SQL

Question

                                                                                             Table 1

T1A	T1B
a	aa
b	bb
c	cc

 

                                                                                           Table 2

T2A	T2B	T2C
a	a	a
b	a	null

How many tuples would the below query output:

Select * from Table1
where T1A = all (Select T2C from Table2 where T2A = 'b');

Answer 1

Subquery:

SELECT T2C FROM Table2 WHERE T2A = 'b';

From Table2, where T2A = 'b', the result is:

T2C
null

 

ALL with NULLs:

When using = ALL (subquery) and the subquery returns NULL, the result of the comparison is UNKNOWN, which behaves like false in SQL's three-valued logic.

So, for each row in Table1, the condition:

T1A = ALL (NULL)

evaluates to UNKNOWN → false.

 

• No rows from Table1 satisfy the condition.

• Output: 0 tuples