table A
userid userip
1 192.168.1.1
null 192.168.1.15
2 192.168.1.13
4 192.168.1.19
null 192.168.1.18table Buserid username grade
1 aa 2
2 bb 3
3 cc 4
4 dd 2
5 ee 6要得到这样的结果
userid userip username grade
1 192.168.1.1 aa 2
null 192.168.1.15 null null
2 192.168.1.13 bb 3
4 192.168.1.19 dd 2
null 192.168.1.18 null null
userid userip
1 192.168.1.1
null 192.168.1.15
2 192.168.1.13
4 192.168.1.19
null 192.168.1.18table Buserid username grade
1 aa 2
2 bb 3
3 cc 4
4 dd 2
5 ee 6要得到这样的结果
userid userip username grade
1 192.168.1.1 aa 2
null 192.168.1.15 null null
2 192.168.1.13 bb 3
4 192.168.1.19 dd 2
null 192.168.1.18 null null
userid userip
1 192.168.1.1
null 192.168.1.15
2 192.168.1.13
4 192.168.1.19
null 192.168.1.18table Buserid username grade
1 aa 2
2 bb 3
3 cc 4
4 dd 2
5 ee 6结果
userid userip username grade
1 192.168.1.1 aa 2
null 192.168.1.15 null null
2 192.168.1.13 bb 3
4 192.168.1.19 dd 2
null 192.168.1.18 null null
LEFT OUTER JOIN table B ON A.userid = B.userid
Left Join B
On A.userid = B.userid