(learn how to use MySQL user-defined variables in SQL statements.)

MySQL User-Defined Variables: A Comprehensive Guide

What are MySQL User-Defined Variables?

In MySQL, user-defined variables allow you to store values during a session and reuse them in subsequent SQL statements. This feature is particularly useful when you need to pass a value obtained from one query into another without hardcoding it.

To define a user-defined variable, use the following syntax:

@variable_name

• @variable_name: The variable name, prefixed with the @ symbol.
• Case Insensitivity: MySQL user-defined variables are case-insensitive. For example, @id and @ID refer to the same variable.

Note: User-defined variables are a MySQL-specific feature and may not be supported in other database systems.

Assigning Values to Variables

MySQL provides two primary methods to assign values to user-defined variables: using the SET statement or the SELECTstatement.

1) Using the SET Statement

The SET statement allows you to assign a value to a variable:

SET @variable_name = value;

Alternatively, you can use the := operator for assignment:

SET @variable_name := value;

Both syntaxes are valid and perform the same operation.

2) Using the SELECT Statement

You can assign a value directly from a query using the SELECT statement with the INTO clause:

SELECT value INTO @variable_name;

Examples of Using MySQL Variables

1) Basic Example

Suppose you want to retrieve the highest price (msrp) from a products table and store it in a variable. You can use the following query:

sql
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SELECT MAX(msrp) INTO @msrp FROM products;

To check the value stored in @msrp, you can run:

sql
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SELECT @msrp;

Output Example:

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+------------------+
| @msrp            |
+------------------+
|           214.30 |
+------------------+

You can now use this variable in other queries. For instance, to find details of the product with the highest price:

sql
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SELECT
    productCode,
    productName,
    productLine,
    msrp
FROM
    products
WHERE
    msrp = @msrp;

Output Example:

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+-------------+--------------------------+--------------+--------+
| productCode | productName              | productLine  | msrp   |
+-------------+--------------------------+--------------+--------+
| S10_1949    | 1952 Alpine Renault 1300 | Classic Cars | 214.30 |
+-------------+--------------------------+--------------+--------+

2) When Query Returns Multiple Values

A MySQL variable can only store a single value. If a query returns multiple rows, MySQL will issue an error, and the variable will store only the first value from the result set.

For example:

sql
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SELECT buyPrice INTO @buy_price
FROM products
WHERE buyPrice > 95
ORDER BY buyPrice;

Error Output:

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ERROR 1172 (42000): Result consisted of more than one row

To handle this, ensure your query returns a single value. For instance, use LIMIT 1:

sql
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SELECT buyPrice INTO @buy_price
FROM products
WHERE buyPrice > 95
ORDER BY buyPrice
LIMIT 1;

You can now check the value stored in @buy_price:

sql
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SELECT @buy_price;

Output Example:

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+------------+
| @buy_price |
+------------+
|      95.34 |
+------------+

Summary of MySQL Variables

• Purpose: User-defined variables allow you to pass data between SQL statements during the same session.
• Syntax: Defined using the @ symbol (e.g., @variable_name).
• Assignment: Use either SET or SELECT INTO for assigning values.
• Limitations:
• Variables can only hold a single value.
• If a query returns multiple rows, the variable will take the first row, and an error may be raised.

By mastering MySQL variables, you can write more dynamic and efficient SQL scripts, especially when dealing with complex queries that require passing data between multiple steps.

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