通俗的讲:
A left join B 的连接的记录数与A表的记录数同
A right join B 的连接的记录数与B表的记录数同
A left join B 等价B right join A
table A:
Field_K, Field_A
1 a
3 b
4 ctable B:
Field_K, Field_B
1 x
2 y
4 zselect a.Field_K, a.Field_A, b.Field_K, b.Field_B
from a left join b on a.Field_K=b.Field_KField_K Field_A Field_K Field_B
---------- ---------- ---------- ----------
1 a 1 x
3 b NULL NULL
4 c 4 z select a.Field_K, a.Field_A, b.Field_K, b.Field_B
from a right join b on a.Field_K=b.Field_KField_K Field_A Field_K Field_B
---------- ---------- ---------- ----------
1 a 1 x
NULL NULL 2 y
4 c 4 z
A left join B 的连接的记录数与A表的记录数同
A right join B 的连接的记录数与B表的记录数同
A left join B 等价B right join A
table A:
Field_K, Field_A
1 a
3 b
4 ctable B:
Field_K, Field_B
1 x
2 y
4 zselect a.Field_K, a.Field_A, b.Field_K, b.Field_B
from a left join b on a.Field_K=b.Field_KField_K Field_A Field_K Field_B
---------- ---------- ---------- ----------
1 a 1 x
3 b NULL NULL
4 c 4 z select a.Field_K, a.Field_A, b.Field_K, b.Field_B
from a right join b on a.Field_K=b.Field_KField_K Field_A Field_K Field_B
---------- ---------- ---------- ----------
1 a 1 x
NULL NULL 2 y
4 c 4 z
解决方案 »
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