The following SELECT statement includes a WITH clause that defines a CTE named planes: With that in mind, let’s start with a simple example. ![]() You can then apply the fundamental principles you learn here to other types of statements as you become more comfortable with how CTEs work. This approach is also a good way to start learning about CTEs without putting any data at risk. For example, you can add a WITH clause to the SELECT query in an INSERT…SELECT statement or in a CREATE TABLE…SELECT statement.įor this article, I focus primarily on creating CTEs that use SELECT statements as their top-level statements because this is the most common way to use a CTE. You can also use CTEs in statements that support the use of SELECT as part of the statement definition. In addition, you can include CTEs with your subqueries when passing them into your outer statements. However, you can also use them with UPDATE and DELETE statements, following the same syntax as shown above. If you don’t specify column names, the column names returned by the SELECT query are used.Ĭommon table expressions are typically used with SELECT statements. If you specify column names, their number must match the number of columns returned by the SELECT query. The CTE name is followed by one or more optional column names, then the AS keyword, and finally a SELECT query enclosed in parentheses. For example, two SELECT statements can include CTEs with the same name because a CTE is limited to the scope of its associated top-level statement. If the WITH clause contains more than one CTE, you must separate them with commas, and you must assign a unique name to each CTE, although this applies only within the context of the WITH clause. In addition, the clause can contain one or more CTE definitions, as shown in the following syntax: The clause precedes the main DML statement, which is sometimes referred to as the top-level statement. Getting started with common table expressionsĪ common table expression is defined inside a WITH clause. Note: the examples use the tables and data that is created in the last section of the article titled: “Appendix: Preparing the demo objects and data”. However, a CTE is neither of these, and in this article, I explain how the CTE works and walk you through a number of examples that demonstrate the different ways you can use them to retrieve data. Another way to think of a CTE is as a type of named subquery that is defined in a clause separate from the main query. One way to think of a CTE is as a type of view with a very limited scope (one statement). But even then, the WITH clause is defined outside of that SELECT statement, and only that SELECT statement can access the result set. In some cases, you can include a CTE with a SELECT statement that is embedded in another statement, as in the case of a subquery or a DELETE…SELECT statement. However, only that statement can access the result set. The WITH clause is associated with a single DML statement but is created outside the statement. A CTE is a named result set that you define in a WITH clause. MySQL also offers another valuable tool for working with data-the common table expression (CTE). You can also access views and temporary tables from within a statement, often along with permanent tables. ![]() ![]() You can join multiple tables in a single query or add subqueries that pull data in from other tables. To see all 11 items in the series, click here.Īs with many relational database management systems, MySQL provides a variety of methods for combining data in a data manipulation language (DML) statement. This article is part of Robert Sheldon's continuing series on Learning MySQL.
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