DQL basic query

1. Basic query collation framework

Be careful :
problem ：SQL In the execution order of , When using having Constraint time , To be able to call select Select the newly set alias in （ This is in contradiction with the specified order ）;
answer ： Generally not in having condition Use in select list Medium alias. however mysql This is extended . stay mysql 5.7.5 Previous version ,ONLY_FULL_GROUP_BY sql mode Not on by default . stay 5.7.5 Or later versions are enabled by default . If ONLY_FULL_GROUP_BY sql mode Don't open , that mysql To the standard SQL The extension of can take effect ： Allow in select list、having condition and order by list Use in does not appear in group by list In the field . here mysql Will randomly choose not to appear in group by list The value of the field in . Effect and use ANY_VALUE() It's the same . Allow in having condition Use in select list Medium alias
Reproduced in :https://www.cnblogs.com/leisurelylicht/p/wei-shen-memysql-having-de-tiao-jian-biao-da-shi-k.html

2.DQL- The basic query

Operator ：

1.1 Arithmetic operator

1.2 Comparison operator

1.3 Logical operators

1.4 An operator

Case list ：

1.5 Sort query

1.6 Aggregate query

1.7 group by Group query

Reference resources

https://www.bilibili.com/video/BV1iF411z7Pu?p=57