And,Where使用提示以及用+进行左关联的提示及注意事项
先左关联后在过滤假如关联的结果里面B.b3=null那么你在where后面在加B.b3=2那么结果中B.b3肯定是没有null的情况的,也就是说用+进行左关联没有用leftjoin灵活待后续看是否有什么好的解决方案
1and和where的滥用用的时候请多注意下面的三段代码的效果肯定是不一样的
(1)
代码如下:
selectA.a1,B.b3
fromAleftjoinB
onA.a2=B.b2
whereA.a3=1
andB.b3=2
先左关联后在过滤假如关联的结果里面B.b3=null那么你在where后面在加B.b3=2那么结果中B.b3肯定是没有null的情况的
(2)
代码如下:
selectA.a1,B.b3
fromAleftjoinB
onA.a2=B.b2
andB.b3=2
whereA.a3=1
关联前先过掉B的b3=2的条件然后再左关联起来那么这个结果可能B.a3会有null的情况的
(3)
代码如下:
selectA.a1,B.b3
fromAleftjoinB
onA.a2=B.b2
andB.b3=2
andA.a3=1
关联前先过掉B中的B3=2和A中的a3=1条件然后再关联起来那么这个结果可能B.b3会有null的情况的
所以大家在用where和and的时候放的位置请多注意了
2用+进行左关联缺陷也许可以有解决方案待后续
(1)
代码如下:
selectA.a1,B.b3
fromA,B
whereA.a2=B.b2(+)
andA.a3(+)=1
andB.b3(+)=2
这个条件应该是和1里面的(1)的情况一样
先左关联后在过滤假如关联的结果里面B.b3=null那么你在where后面在加B.b3=2那么结果中B.b3肯定是没有null的情况的
也就是说用+进行左关联没有用leftjoin灵活待后续看是否有什么好的解决方案
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