Simple =, join, left out join, right Outer Join, cross join

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 in Table B, then it will be cleared.
B. Left join
Select a. a, B.
From Table
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..

E. g Table A Table B
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
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