Problem

Given two tables, "A" and "B", write a SQL query to find all the rows from table "A" that have no corresponding rows in table "B".
Input:
Table A:
id | data
---|------
1  | abc
2  | def
3  | ghi
Table B:
id | data
---|------
2  | def
3  | ghi
4  | jkl
Output:
Table A:
id | data
---|------
1  | abc

Solution

SELECT * FROM A
LEFT JOIN B ON A.id = B.id
WHERE B.id IS NULL;

/*

The above solution is optimal because it uses a LEFT JOIN, which will return all rows from table A, even if there is no corresponding row in table B.

*/