Overcoming MySQL Error 1204: Correcting SET Command Misuse

When working with MySQL, you might come across Error 1204 – SQLSTATE: HY000 (ER_SET_CONSTANTS_ONLY), which indicates an issue with the usage of the SET command in your SQL statements. This error message means that a non-constant expression was used where MySQL expects a constant value. Understanding how to properly use SET and correcting any misuse is key to resolving this error. Let’s dive into the various scenarios that can lead to this error and how to fix them.

Understanding the Error

MySQL’s SET command is used to assign values to variables, system variables, or certain operation options. Error 1204 is raised when an attempt is made to assign a non-constant expression to a user-defined variable or one of the few system variables that require a constant expression.

Diagnosing the Problem

To diagnose this error, examine the SET statement that is causing the issue. Look for any functions, subqueries, or variables being assigned to the variable instead of a constant value.

Fixing the Error

Let’s look at some examples of incorrect usage of the SET command and how to correct them.

Example 1: Using a Function

Incorrect usage:

SET @myVar = NOW();

To fix this, you should directly use the function where needed in your queries, or if you need to capture the current timestamp for reuse in a session, you can assign it to a session variable:


Example 2: Using a Subquery

Incorrect usage:

SET @userID = (SELECT id FROM users WHERE username = 'johndoe');

To fix this, you could use the subquery directly in your SQL statements where needed, or if you need to capture a single value for reuse, make sure the subquery returns a single value and assign it like this:

SELECT id INTO @userID FROM users WHERE username = 'johndoe' LIMIT 1;

Example 3: Using a Variable in a SET Statement

Incorrect usage:

SET @a = 10;
SET @b = @a + 1;

In this case, the second line is incorrect because it uses another variable in the assignment. To fix this, you can perform the operation in a SELECT statement:

SET @a := 10;
SELECT @a + 1 INTO @b;

Preventing Future Errors

To avoid Error 1204 in the future, remember the following guidelines when using the SET command:

  • Use constant values for assignments with SET.
  • Avoid using functions, subqueries, or variables when setting a value with SET.
  • Familiarize yourself with which system variables require constant expressions.


Error 1204 in MySQL is a sign that there is a misuse of the SET command in your SQL statements. By understanding the correct usage of SET and revising your code accordingly, you can resolve this error and ensure your database operations run smoothly. Always test your SQL statements to verify their correctness and consult the MySQL documentation for best practices and detailed explanations of command usage.

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