SQL AND OR 运算符

Question

Why this below query ONLY brings deptno = 10 ?
Why is not not bringing rows of job IN ('MANAGER','SALESMAN') and deptno = 30 results?

select * 
from emp 
where deptno=10 
  and ((job IN ('MANAGER','SALESMAN') and deptno = 30) or (1=1));

Answer 1

(1=1) is always TRUE .
So any condition/boolean expression combined with OR with that condition, like:

(job IN ('MANAGER','SALESMAN') and deptno = 30) or (1=1)

will also be always TRUE .
This makes your WHERE clause equivalent to:

where deptno=10 and TRUE

and simpler:

where deptno=10

If you want the rows of the table of deptno=10 and the rows of deptno=30 only if job IN ('MANAGER','SALESMAN') then you should have:

where deptno=10 or ((job IN ('MANAGER','SALESMAN') and deptno = 30) 

Answer 2

You probably wanted to do this:

select * 
from emp 
where (deptno=10 )
  or ((job IN ('MANAGER','SALESMAN') and deptno = 30));