MySQL Cookbook

Using SQL Variables in Queries

Problem

You want to save a value from a query so you can refer to it in a subsequent query.

Solution

Use a SQL variable to store the value for later use.

Discussion

As of MySQL 3.23.6, you can assign a value returned by a SELECT statement to a variable, then refer to the variable later in your mysql session. This provides a way to save a result returned from one query, then refer to it later in other queries. The syntax for assigning a value to a SQL variable within a SELECT query is @var_name := value, where var_name is the variable name and value is a value that you’re retrieving. The variable may be used in subsequent queries wherever an expression is allowed, such as in a WHERE clause or in an INSERT statement.

A common situation in which SQL variables come in handy is when you need to issue successive queries on multiple tables that are related by a common key value. Suppose you have a customers table with a cust_id column that identifies each customer, and an orders table that also has a cust_id column to indicate which customer each order is associated with. If you have a customer name and you want to delete the customer record as well as all the customer’s orders, you need to determine the proper cust_id value for that customer, then delete records from both the customers and orders tables that match the ID. One way to do this is to first save the ID value in a variable, then refer to the variable in the DELETE statements:^[5]

mysql> SELECT @id ...