Mysql Hierarchical Query Self Join

;) > How does one create a self-join in MySQL tables? I've only played a little > with SQL selects so far, some inner joins, and now I have to create a > many-to-many relationship between records in the same table. Given the following Employees table: Note that this table has a certain hierarchy of employees and managers, such that a certain employee may also be a manager. After migration, users will have a compact and easy to understand hierarchical representation. Hope you will get the result. As I said, there are a few keywords we'll need to get acquainted with in order to understand hierarchical querying. Specifically, I will explore the two most popular hierarchical data models:. Once you have this table, hierarchical queries become very easy and fast. A self join uses the inner join or left join clause. Simplest way to do a recursive self-join? Ask Question Asked 9 years, 10 months ago. Test: Quiz: Self Joins and Hierarchical Queries Review your answers, feedback, and question scores below. Notice that the first query is a join, but the second query is a dependent (correlated) subquery, where the subquery refers to values in the enclosing query. It seems to be lightning-fast, with no arbitrary depth-of-query limit. My guess is, JOIN syntax will never cease to be an object of discussion. Edureka's Data Science Training in Noida allows you to acquire knowledge using R in machine learning algorithms such as K-Means Clustering, Decision Trees, Random Forest, and Naive Bayes. A self join is useful for when you want to combine records in a table with other records in the same table that match a certain join condition. Returns records where there is at least one row in both tables that match the join condition. Because the same table appears twice in a single query, we. Processing hierarchical data is one of the most frequent applications of self joins. They are special cases of more general recursive fixpoint queries, which compute transitive closures. It was just a brief overview. A self join uses the inner join or left join clause. Refer to the image below. Our special concentration would be over. Hierarchical queries MUST use the LEVEL pseudo column. The net result of this according to SQL profiler is that it pulls all 20,000 records for the name query and all 50,000 records for the state query and then appears to merge them in PowerQuery. hierarchies are just based on self-joins ! Given that, in some circumstances, temporary tables are just files (if MyISAM) written to the disk (in the /temp directory), and that in this case i don't see any reason to forbid the self join, it would be nice to have a syntax to direct to the disk the temporary tables and allow the self join. 1 now features SQL-standard CTE syntax; more information is here; the entry below, from 2013, shows how to work around the absence of CTEs in older MySQL versions. 1) Presentation of Hierarchical data. Oracle processes hierarchical queries as follows: A join, if present, is evaluated first, whether the join is specified in the FROM clause or with WHERE clause predicates. descendant WHERE cl. A quick reminder: hierarchical data is a parent-child relationship contained in one table. Self Join is one of the important technique to solve many SQL query related problem where two columns of the table contain the same type of data e. More specifically, the INNER JOIN is for when you're only interested in returning the records where there is at least one row in both tables that match the join condition. I read an article that suggested avoiding self joins by using stored procs. Please use the below query. A great starting point for this lab is to review the create_mysql_store. The top-query sets the starting row, and the second query recursively navigates the tree. COURSE DESCRIPTION: Structured Query Language (SQL) is the language used to manipulate data in relational databases. Every MySQL database programmer should learn and apply the newly added MariaDB and MySQL Window Functions and Common Table Expressions(CTEs) in their daily work. SQL HOME SQL Intro SQL Syntax SQL Select SQL Select Distinct SQL Where SQL And, Or, Not SQL Order By SQL Insert Into SQL Null Values SQL Update SQL Delete SQL Select Top SQL Min and Max SQL Count, Avg, Sum SQL Like SQL Wildcards SQL In SQL Between SQL Aliases SQL Joins SQL Inner Join SQL Left Join SQL Right Join SQL Full Join SQL Self Join SQL. Each join goes "down" a level from the node above it. SELECT on the table with the self-join relationship, then query the OQGRAPH table by joining it back to your original table. Once you have this table, hierarchical queries become very easy and fast. depth > 0 Of course, there is a big downside whenever you use denormalized data like this. Self Join has always been an note-worthy case. EntityParent = e2. CONNECT_BY_ROOT : Returns the root node(s) associated with the current row. A self-join is a query in which a table is joined (compared) to itself. SQL to create and populate. In standard SQL:1999 hierarchical queries are implemented by way of recursive common table expressions (CTEs). MySQL LEFT JOIN. Theta-style. It is relatively easy to migrate from this representation to one using hierarchyid. ancestor = 20 AND cl. However, an inline view is a subquery with a twist. SQL JOINs are often misunderstood and one of the biggest causes of database optimization problems. Web programmers use hierarchical data for a number of applications, including content management systems, forum threads, mailing lists, and e-commerce product categorization. hierarchical related issues & queries in DbaXchanger. Question: Given an Employee table which has 3 fields - Id (Primary key), Salary and Manager Id, where manager id is the id of the employee that manages the. Since the category hierarchy can be arbitrarily deep, no fixed set of SQL query or join operations can solve what is a recursive problem. What is Self Join in SQL? A self join is a join in which a table is joined with itself (which is also called Unary relationships), especially when the table has a FOREIGN KEY which references its own PRIMARY KEY. MySQL joins: ON vs. This usually happens when the matching column or WHERE condition is not specified. Creating Simple Queries. Visually explore and analyze data—on-premises and in the cloud—all in one view. In this tutorial , I will explain how to manage hierarchical data using MySQL and PHP. The UNION [ALL], INTERSECT, MINUS Operators. Hope you will get the result. select * from Items i inner join CategoriesFlat c on c. In this article, however, we will explain how to join records from the same table. Use Nested model: But if you convert the data structure your data structure to Nested set model then you can still use the self join to get the tree. To display the hierarchy, we must first retrieve it. Here, we have to use Self-Join as the requirement wants us to analyze the EmployeeDetails table as two different tables, each for Employee and manager records. Closely related to self-joins are hierarchical queries. So, the query language is basically the MongoDB query language. However, there is a special case that you need to join a table to itself, which is known as a self join. To query the whole organization structure including the CEO, you need to use the LEFT JOIN clause rather than the INNER JOIN clause as the following query:. Here's a great slideshow about SQL anti-patterns and solutions to them. May 07, 2007 01:24PM Re: Optimising query with self-join and GROUP BY. In the above example, we retrieve all customers whose last name matches the first name of. Dear Michail and Sergey, Thank you very much for your responses and kind suggestions! On 29. (Hierarchical data in SQL is an anti-pattern, but don't be disheartened - we all run into this one) more. It is useful for querying hierarchical data or comparing rows within the same table. Edureka's Data Science Training in Noida allows you to acquire knowledge using R in machine learning algorithms such as K-Means Clustering, Decision Trees, Random Forest, and Naive Bayes. Note that referencing the same table more than. Research on hierarchical tree structure of multi. In this video we will learn about 1. MySQL is a popular open-source relational database management system (RDBMS) that is developed, distributed and supported by Oracle Corporation. The net result of this according to SQL profiler is that it pulls all 20,000 records for the name query and all 50,000 records for the state query and then appears to merge them in PowerQuery. Test: Quiz: Self Joins and Hierarchical Queries. Joining Three or More Tables in SQL With MySql (Nested Joins) August 4, 2011 | 6 Comments. Third, the query that uses the employee_paths CTE joins the result set returned by the CTE with the offices table to make the final result set. A single query combines two other queries which select ancestors and descendants in a UNION ALL and order them properly using a session variable. Hierarchical Data. Outer joins and self-joins help you perform more-complex actions to obtain results that are difficult to get with an ordinary equijoin. Common table expressions are an optional part of the syntax for DML statements. In the following examples, you will learn how to harness the power of a recursive CTE query by fulfilling a common business requirement, retrieving hierarchical data. To query the whole organization structure including the CEO, you need to use the LEFT JOIN clause rather than the INNER JOIN clause as the following query:. Any thoughts? • using a subquery/self-join to. SQL join three or more tables based on a parent-child relationship Last update on September 19 2019 10:37:27 (UTC/GMT +8 hours) In this page, we are going to discuss such a join which involves the participation of three tables and there is a parent-child relationship between these tables. ExcelR offers an interactive instructor-led 160 hours of virtual online Data Science certification course training in Kenya, the most comprehensive Data Science course in the market, covering the complete Data Science life cycle concepts from Data Extraction, Data Cleansing, Data Integration, Data Mining, building Prediction models and Data. You can use multiple tables in your single SQL query. This chapter describes SQL queries and subqueries. A self join is useful for comparing rows within a table or querying hierarchical data. It seems to be lightning-fast, with no arbitrary depth-of-query limit. depth > 0 Of course, there is a big downside whenever you use denormalized data like this. They are defined using a WITH clause:. A significant contribution to the long running nature of the CTE query is this portion of the query plan that estimates less than 10 rows on the outer and inner query, but ends up having to iterate through millions of rows on both sides of the query. Hope you will get the result. To query the whole organization structure including the CEO, you need to use the LEFT JOIN clause rather than the INNER JOIN clause as the following query:. 2009, at 16:53, Sergey Petrunya wrote: > this makes it clear that index on O1. LEFT JOIN Categories AS C1 ON C. Interested in functions, hooks, classes, or methods? Check out the new WordPress Code Reference!. Using hierarchical data in a SQL database is very common. Introduction Hierarchical query is a type of SQL query that is commonly leveraged to produce meaningful results from hierarchical data. Using Joins at the. What you're left with is: use an alternate implementation of hierarchical data structure (answers to this question might be a good reference on this) build self join queries with a depth limit. Because of its simplicity, the adjacency list model is a very popular choice by developers and database administrators. Theta-style. Ask Question Let me know how to achieve the same using a single MySQL query if possible. Recursive joins are highly characteristic of hierarchical data, and therefore become a serious issue with XML data. In addition, the join is based on the manager ID in the Employees table and the employee ID in the CTE. Answering questions asked on the site. Hierarchical Queries with Common Table Expressions I wanted a simple example of a hierarchical query using a common table expression in SQL Server. The tables are described below This table describes a hierarchy of requirements ie. I'm a SQL newbie so unless the answer is specific, I could be missing it. Self-joins are used to compare values in a column with other values in the same column in the same table. This type of join is known as the self-join. Re: self-joins in hierarchical queries: optimization problem View as plain text Hi Olga, On Thu, Oct 29, 2009 at 03:29:58PM +0000, Olga Lyashevska wrote: > I have a table which contains taxonomic data (species, genera, family, > order, class) and it is organized as adjacency list model. Simulating a self-driving car user interface, clustering and query expansion. Course Tutorial Site. MSDN had an example called Recursive Queries Using Common Table Expressions , but it relied on the Adventure Works database and was joining across three different tables. The simplest Join is INNER JOIN. A relational database consists of multiple related tables linking together using common columns which are known as foreign key columns. tsn will not be useful. However, there is a special case that you need to join a table to itself, which is known as a self join. You should note that the CTE self-references itself from inside the second query. Ask Question Let me know how to achieve the same using a single MySQL query if possible. * FROM categories_closure AS cl INNER JOIN categories AS cat ON cat. Oracle SELF Join: A self join is a join in which a table is joined with itself. Query Language. Because the query that uses self join references the same table, the table alias is used to assign different names to the same. Please use the below query. In the previous installment of this MySQL series, I demonstrated how you could fetch data using MySQL clauses. The self join is used to join a table to itself when using a join. APPLIES TO: SQL Server Azure SQL Database Azure SQL Data Warehouse Parallel Data Warehouse The built-in hierarchyid data type makes it easier to store and query hierarchical data. We have a employee table and we want to specify the employee name , manager name for all the employee. In standard SQL, they are not equivalent. Self-joins are used to compare values in a column with other values in the same column in the same table. Unlike a derived table, a CTE can be self-referencing (a recursive CTE) or can be referenced multiple times in the same query. the query just uses as many self-joins as levels. Self Joins. ] If you have been using certain DBMSs, or reading recent versions of the SQL standard, you are probably aware of the so-called "WITH clause" of SQL. This is useful for hierarchical relationships such as parts lists, employee to supervisor reporting, customer referrals, etc. Finally, combine all result sets 1, 2 and 3 using the UNION ALL operator; Using MySQL recursive CTE to traverse the hierarchical data. After migration, users will have a compact and easy to understand hierarchical representation. The query in meta_form can be incredibly slow on sites with a lot of post meta. On the previous pages you saw how you can use self-joins to see who is someone's direct manager. In this lesson, you will start with a new table, and insert data by using the hierarchical methods. Natural Join: Guidelines - The associated tables have one or more pairs of identically named columns. Often the data you need will be stored in multiple tables. name as t1, t2. Both CTEs and window functions enable easy solutions to many query challenges that in prior releases have been difficult and sometimes impossible to surmount. I want to learn more about this. 2009, at 16:53, Sergey Petrunya wrote: > this makes it clear that index on O1. After all children are found, the first query moves to the next element in the table and recursively searches for its children. Introduction Most users at one time or another have dealt with hierarchical data in a SQL database and no doubt learned that the management of hierarchical data is not what a relational database is intended for. It only exists inside of the FROM clause as a run-time result set. Step-by-Step Guide to Creating SQL Hierarchical Queries. Flexibility of Schema. , the CEO, does not report to anyone in the company, therefore, the reportTo column contains the NULL value. An inline view is basically a query inside another query, which makes it a subquery. To query hierarchical data, you will need to refer to a family of keywords and build a hierarchical query clause. Hierarchical queries are useful in reporting purpose and when user wants to fetch the data from hierarchical structure these queries are very useful. This is an. What you're left with is: use an alternate implementation of hierarchical data structure (answers to this question might be a good reference on this) build self join queries with a depth limit. Justin Swanhart. We have a kind of a condition in this query, that makes it our starting point for the rest of the data fetching query. We join a table to itself to evaluate the rows with other rows in the same table. (This is not strictly true if you are willing to accept some de-normalization of the self-referential tables. aliased that can shorten the verbosity self- referential joins, at the expense of query flexibility. using a subquery/self-join to recursively retrieve a tree: but not sure how to do it as a mysql query. We have a President also who does nor report to anybody to show in the query. A great starting point for this lab is to review the create_mysql_store. python or R code call to mySQL for hierarchical and recursive queries. Section 3 Lesson 4: Self Joins and Hierarchical Queries Objective Construct and execute a SELECT statement to join a table to itself using a self-join Interpret the concept of a hierarchical query Create a tree-structured report Format hierarchical data Exclude branches from the tree structure Vocabulary Identify the vocabulary word for each. Cameron DeWitt. Using Joins at the. Practice #1: Use self-join for employees table. In many small transactions, such as those affecting only a small set of rows, index nested loops joins are superior to both merge joins and hash joins. LEVEL : The position in the hierarchy of the current row in relation to the root node. Data Science Training includes a conceptual understanding of Statistics, Time Series, Text Mining and an. Like other relational systems, MySQL stores data in tables and uses structured query language (SQL) for database access. Introduction to SQL self-join. Given the following Employees table: Note that this table has a certain hierarchy of employees and managers, such that a certain employee may also be a manager. categoryId where c. In concept this is similar to creating a temporary table and then using the temporary table in your query, but the approach is much simpler, because it can all be done in one step. A great starting point for this lab is to review the create_mysql_store. Note that the SQL needs to end with semi-colon if you have multiple queries in the query window. EXISTS in MySQL is optimized so that it returns as soon as the first value is found. - hierarchy. tMap has another level of joining capability, in that it can join together data in a hierarchical fashion. Using Joins at the. PostgreSQL provides the WITH statement that allows you to construct auxiliary statements for use in a query. Mysql self join keyword after analyzing the system lists the list of keywords related and the list of websites with related content, in addition you can see which keywords most interested customers on the this website. Use the recursive CTE… [code]DECLARE @TABLE TABLE ( Type1 NVARCHAR(10) ,Type2 NVARCHAR(10) ) INSERT INTO. It is performed whenever you need to fetch records from two or more tables. This language is simple and consists of mainly DDL, DML DCL & TCL commands to retrieve and manipulate data. Using Subqueries. The tables are described below This table describes a hierarchy of requirements ie. Unnesting of Nested Subqueries. Natural Join: Guidelines - The associated tables have one or more pairs of identically named columns. Query Language. I needed to setup a self-referencing table in order to store some hierarchical. Here, in this post, we will try to manage data with hierarchical relation or parent-child relation of a specific table in SQL server. One practical use for self-joins: obtaining running counts and running totals in an SQL query. Any remaining WHERE clause predicates are evaluated. In fact, it can be classified under any type of. The query begins at the root nodes. So, the next time you (and I) see a self referencing table, we will not need to shy away. Common applications of recursive CTEs include series generation and traversal of hierarchical or tree-structured data. To complete this tutorial, you need SQL Server Management Studio, access to a server that's running SQL Server, and an AdventureWorks database. Copy and paste the following SQL to your SQLyog free Community Edition query window. Anchor query (runs once and the results ‘seed’ the Recursive query) Recursive query (runs multiple times and is the criteria for the remaining results) UNION ALL statement to bind the Anchor and Recursive queries together. This chapter contains these sections: About Queries and Subqueries. Parent requirement. First Way: We would use self Join, to join the employee table to itself like this : select e1. Show Generations of each row Find all possible parents of a specific row Find all possible childs of a specific row Show all. Mysql self join keyword after analyzing the system lists the list of keywords related and the list of websites with related content, in addition you can see which keywords most interested customers on the this website. Summary: this tutorial shows you how to join table to itself by using the SQL self-join technique. Write a SQL query to fetch all the Employees who are also managers from EmployeeDetails table. MySQL also supports nested joins. Web programmers use hierarchical data for a number of applications, including content management systems, forum threads, mailing lists, and e-commerce product categorization. The net result of this according to SQL profiler is that it pulls all 20,000 records for the name query and all 50,000 records for the state query and then appears to merge them in PowerQuery. You should try to have most, if not all, of your queries to be covered. SELECT * FROM T1 LEFT JOIN (T2,T3) ON (T2. A self join uses the inner join or left join clause. Customers have orders, which have line items, which refer to products, which have prices. The relational part comes from entities referencing other entities in what is known as one-to-many relationship, like a traditional hierarchical model, and many-to-many relationship, like a navigational (network) model. WITH MyCTE. A self join is a join of a table to itself. employee_name,m. The table is not actually copied, but SQL performs the command as though it were. So, the query language is basically the MongoDB query language. To perform the self-join, we use either an inner join or left join clause. SQL HOME SQL Intro SQL Syntax SQL Select SQL Select Distinct SQL Where SQL And, Or, Not SQL Order By SQL Insert Into SQL Null Values SQL Update SQL Delete SQL Select Top SQL Min and Max SQL Count, Avg, Sum SQL Like SQL Wildcards SQL In SQL Between SQL Aliases SQL Joins SQL Inner Join SQL Left Join SQL Right Join SQL Full Join SQL Self Join SQL. Inner Join: Consider we need to find the teachers who are class teachers and their corresponding students. SQL self join with left join example. Open the job jo_cook_ch04_0080_hierarchicaltMapJoin. However, notice that I join the Employees table to the CTE itself. Let's take a look at an example where we query the Northwind database to try to find out how many customers fall into various categories based on sales. Since you are using MySQL you can't use common table expressions (the WITH statement and it's RECURSIVE modifier) present in other RDBMSes. In the above example, we retrieve all customers whose last name matches the first name of. Note that. However, an inline view is a subquery with a twist. Repeated Self-joins. A hierarchical query is a type of SQL query that handles hierarchical model data. In order to query hierarchical data, more established RDBMSs such as Oracle. This simple example shows how easily this can be achieved using tMap. And read MySQL: Tree-Hierarchical query - Grijesh Chauhan Oct 8 '13 at 9:24. Because of this, SELECT … - Selection from MySQL Cookbook [Book]. Anchor query: This query is the starting point that acts as a base to enable the process of recursion in the CTE. categoryId where c. Terdapat tiga macam JOIN yaitu: INNER JOIN, LEFT JOIN, dan RIGHT JOIN. * FROM categories_closure AS cl INNER JOIN categories AS cat ON cat. The simplest Join is INNER JOIN. Managing Hierarchical Data with PHP and MySQL; is Hierarchical Data? is a little more complex and requires the addition of another self join and a sub-query. A self join uses other joins such as inner join and left join. Retrieving hierarchy at all levels - Self joining table. Tipe join ini akan mengambil semua row dari table asal dan table tujuan dengan kondisi nilai key yang terkait saja, dan jika tidak maka row tersebut tidak akan muncul. To complete this tutorial, you need SQL Server Management Studio, access to a server that's running SQL Server, and an AdventureWorks database. I found a few methods but I would like to know which one is the best. Left, right, outer, inner, cross join. MySQL joins: ON vs. Left, right, outer, inner, cross join. Anchor query (runs once and the results ‘seed’ the Recursive query) Recursive query (runs multiple times and is the criteria for the remaining results) UNION ALL statement to bind the Anchor and Recursive queries together. To query the whole organization structure including the CEO, you need to use the LEFT JOIN clause rather than the INNER JOIN clause as the following query:. We join a table to itself to evaluate the rows with other rows in the same table. Note that the left and right table of the join keyword must both return a common key that can be used for the join. Prerequisites. Hierarchical Queries. Tipe join ini akan mengambil semua row dari table asal dan table tujuan dengan kondisi nilai key yang terkait saja, dan jika tidak maka row tersebut tidak akan muncul. Once you have this table, hierarchical queries become very easy and fast. Hierarchical queries MUST use the LEVEL pseudo column. When a query can be answered completely by the data stored in the index, the query said to be covered by the covering index. Example: SELECT * FROM a JOIN b ON TRUE, b JOIN c ON TRUE; The query above is equivalent to. join() also includes a feature known as Query. python or R code call to mySQL for hierarchical and recursive queries. CategoryID. After all children are found, the first query moves to the next element in the table and recursively searches for its children. Eliminating Duplicates from a Self-Join Result Problem Self-joins often produce rows that are "near" duplicates—that is, rows that contain the same values but in different orders. Justin Swanhart. The self join is often used to query hierarchical data or to compare a row with other rows within the same table. Hierarchical queries : I have explained lot of advanced sql topics like materialized view in sql , partitioning techniques in oracle. A self join could just as well be an outer join or an inner join - it just depends on how the query is written. - hierarchy. Course Tutorial Site. which may ask in interview with examples. It was just a brief overview. In this lesson, you'll learn to create reports from two or more tables based on data in one of those tables or even in a separate table altoget. INNER JOIN adalah tipe join yang akan kita bahas pertama. They are special cases of more general recursive fixpoint queries, which compute transitive closures. Outer joins and self-joins help you perform more-complex actions to obtain results that are difficult to get with an ordinary equijoin. The top manager i. A "self join" means that you are going to start with one table, and you are going to join that same table to itself. So, the next time you (and I) see a self referencing table, we will not need to shy away. Including nested JSON objects in your result set is a great way to accomplish this naturally. Anchor query (runs once and the results ‘seed’ the Recursive query) Recursive query (runs multiple times and is the criteria for the remaining results) UNION ALL statement to bind the Anchor and Recursive queries together. To join a table itself means that each row of the table is combined with itself and with every other row of the table. It seems to be lightning-fast, with no arbitrary depth-of-query limit. Because of this, SELECT … - Selection from MySQL Cookbook [Book]. An asterisk (*) indicates a correct answer. Dear Michail and Sergey, Thank you very much for your responses and kind suggestions! On 29. To perform a self join, you must use table aliases to not repeat the same table name twice in a single query. Any thoughts? • using a subquery/self-join to. Sometimes, we call this subquery is a plain subquery. Hierarchical Data (SQL Server) 10/04/2019; 13 minutes to read; In this article. Here, in this post, we will try to manage data with hierarchical relation or parent-child relation of a specific table in SQL server. We have a kind of a condition in this query, that makes it our starting point for the rest of the data fetching query. It is used to retrieve data from multiple tables. These statements are often referred to as common table expressions or CTEs. You can use JOINS in the SELECT, UPDATE and DELETE statements to join the MySQL tables. A very rudimentary database engine which is loosely based on a hybrid between MySQL and SQLite. This keyword will create the result-set by combining all rows from both the tables where the condition satisfies i. You could do a 4-way self join to get the 4 different levels I really think that would be a more efficient solution than a hierarchical query. Any remaining WHERE clause predicates are evaluated. This simple example shows how easily this can be achieved using tMap. Me • 20+ years experience • Application/SDK developer • Support, Training, Proj Mgmt • C, Java, Perl, PHP • SQL maven • Zend Framework 1. Extract hierarchical relationships within a table. The below query with self join with outer join to extract the data in the desired format. This work focuses on the integration of tree-like hierarchical data or information that when modeled as a relational schema can take the shape of a flat schema, a self-referential schema or a. What you're left with is: use an alternate implementation of hierarchical data structure (answers to this question might be a good reference on this) build self join queries with a depth limit. Returns data when columns match other columns in same table. COURSE DESCRIPTION: Structured Query Language (SQL) is the language used to manipulate data in relational databases. Traversing a hierarchical tree could be a very complex and tedious process if it were not for Oracle's internal operators and pseudocolumns. MySQL also supports nested joins. Web programmers use hierarchical data for a number of applications, including content management systems, forum threads, mailing lists, and e-commerce product categorization. A self-join is a query in which a table is joined (compared) to itself.