How to Correctly Use the 'NOT IN' Operator in MySQL Queries?

MySQL “NOT IN” conditional query detailed explanation

MySQL database supports the "NOT IN" operator, which is used to retrieve rows from one table whose specified column values ​​do not exist in another table. However, it should be noted that the syntax of "NOT IN" query in MySQL is different from other database systems.

Grammar error analysis

The syntax you are trying to use:

<code class="language-sql">SELECT * FROM Table1 WHERE Table1.principal NOT IN Table2.principal</code>

produces a syntax error because you are trying to compare two columns directly without using a subquery.

Correct syntax for "NOT IN" query

The correct syntax for “NOT IN” query in MySQL is as follows:

<code class="language-sql">SELECT * FROM Table1 WHERE Table1.principal NOT IN (SELECT principal FROM Table2)</code>

In this syntax, we use a subquery to select values ​​from the “principal” column of Table2 and compare these values ​​to the “principal” column of Table1. The result will be a list of rows in Table1 where the "principal" value is not in Table2.

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