Cancel SQL select 1

Select sum (1) From mytable

If the table contains 100 records, 100 constant 1 is returned.

It is generally used as a condition. When used as a condition:
Select 1 from authors
And
Select * from authors is equivalent

 

Delete from table_a
Where not exists (select 1 from table_ B where table_ B .xh = table_a.xh)
And XH in (select XH from table_a where table_a.lxh = ac_lxh );

Select 1 from ...... Is this method used to check whether there are records in the target table?

 

Exists (select 1 from table_ B where table_ B .xh = table_a.xh)
This is only to check whether table_ B .xh = table_a.xh exists.
Select 1 can also be select 0, select *, select 'A', etc.

Check whether there are any associated records.

Check whether the returned result is true.
It is selected only if it is related.