Nuances in SQL not in and not exists usage

The above two simple SQL statements show that the final results of the query should be the same, but the actual results are different from what we think.

The result of the first SQL query contains a piece of data.

The second SQL query result is empty.

Cause:

For subqueries not exists, the query results of rows not returned by subqueries are different from those returned by subqueries.

These small differences must not be ignored by us. Once the project is large, it takes considerable time to track specific errors. Try to put unnecessary errors in the cradle.

It's awkward.

The above two simple SQL statements show that the final results of the query should be the same, but the actual results are different from what we think.

The result of the first SQL query contains a piece of data.

The second SQL query result is empty.

Cause:

For subqueries not exists, the query results of rows not returned by subqueries are different from those returned by subqueries.

These small differences must not be ignored by us. Once the project is large, it takes considerable time to track specific errors. Try to put unnecessary errors in the cradle.

It's awkward.