Why Does 'Unknown Column in Where Clause' Occur When Using Aliases in SQL Queries?

SQL error: "Unknown Column in Where Clause"

When executing a SQL query similar to:

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

You may encounter "Unknown Column 'user_name' in where clause" error. This error is because the SQL parsing order is from right to left.

Specifically, the WHERE clause is parsed before the SELECT clause. When the WHERE clause is parsed, the alias "user_name" has not yet been applied to the "u_name" column. Therefore, the database interprets "user_name" as an unknown column and throws an error.

To solve this problem, you can use the original column name "u_name" in the WHERE clause:

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

Alternatively, you can use a subquery to alias the column before using the alias in the WHERE clause:

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

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