Why Does My SQL Query Fail with 'Unknown Column In Where Clause' When Using Aliases?

SQL query causes "Unknown Column In Where Clause" error due to alias

Question:

Queries using aliases in the SELECT statement will cause an error, prompting an unknown alias column in the WHERE statement. For example, the following query triggers this error:

<code class="language-sql">SELECT u_name AS user_name FROM users WHERE user_name = "john";</code>

Explanation:

The execution order of SQL is from right to left. In this example, the WHERE clause is executed before the SELECT clause. Therefore, when the WHERE clause is parsed, the alias user_name has not been defined yet.

Solution:

To solve this problem, you can use the original column names in the WHERE clause:

<code class="language-sql">SELECT u_name AS user_name FROM users WHERE u_name = "john";</code>

Or enclose the alias column name in parentheses in the WHERE clause:

<code class="language-sql">SELECT u_name AS user_name FROM users WHERE (user_name = "john");</code>

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