a. =
eg.
Select A.a B.b from A, B where A.a=B.a and A.c=’herengang’;
under this condition, it only shows the data that A.a=B.a and A.c=”herengang”.
Although it there is data which .c is “herengang” ,but if we can’t find such record which .a value equals to A.a in Table B, then it will be cleared.
b. left join
select A.a, B.a
from table A
left join table B
on A.a =B.a and A.c=”herengang”
under this condition, all of data which .c is “herengang” will be retrieved, and then if B.a exists, then show the value of B.a .
e.g Table A Table B
columnA columnB columnC columnA columnB columnC
1 3 a 1 'ok' 23.3
2 4 b 3 'sorry' 3.0
A join B (A inner B) ,result is
1 3 a 1 'ok' 23.3
A left join B, result is
1 3 a 1 'ok' 23.3
2 4 b null null null
A cross join B
1 3 a 1 'ok' 23.3
1 3 a 3 'sorry' 3.0
2 4 b 1 'ok' 23.3
2 4 b 3 'sorry' 3.0