HOW TO USE JOIN

About join

Left Join ,Right Join ,Inner join

Join queries is the main query style in relational database 

1.Inner join is based on the total value of the column to each table match their rows.

2.Left Join is based on all rows of the left table, not just the ones in which the joined columns match.

3.Right Join is based on all rows of the left table, not just the ones in which the joined columns match.

example:

1.

table stu:

id     name    age 

1     'wang'   20  

2     'li'     21     

4     'liu'    22   

5     'yang'   22  

6     'lu'     22

table mark:

id     mark    stuid

1      61      2

2      44      2

3      66      3

2.

Inner join result:

select s.name,m.mark from stu s , mark m where
s.id=m.stuid;

NAME

--------------------------------------------------------------------------------

      MARK

----------

li       44

li       61

Right Join result:

select s.name,m.mark from stu s , mark m where
s.id(+)=m.stuid;

NAME

--------------------------------------------------------------------------------

      MARK

----------

li      44

li      61

       66

Left Join result:

select s.name,m.mark from stu s , mark m where
s.id=m.stuid(+);

NAME

--------------------------------------------------------------------------------

      MARK

----------

li       61

li      44

lu

NAME

--------------------------------------------------------------------------------

      MARK

----------

yang

liu

wang

if the left  table row has no matching row in the right table  or the right  table row has no matching row in the left table,the result is null.