sql的 left join连接查询
2014-02-12 16:43
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left join的条件on和where
使用left join 的时候,如果A表和B表中关联的条件不写在on条件后,而写在where条件后,会报语法错误,
一般传统的sql语句的执行顺序是:from-->where-->group by --> having--> order by --> limit;
如果使用left join之后,left join是在from范围内,所以先on条件筛选表,然后两表再做left join,而对where来说,是对left join结果之后的再次筛
使用left join 的时候,如果A表和B表中关联的条件不写在on条件后,而写在where条件后,会报语法错误,
一般传统的sql语句的执行顺序是:from-->where-->group by --> having--> order by --> limit;
如果使用left join之后,left join是在from范围内,所以先on条件筛选表,然后两表再做left join,而对where来说,是对left join结果之后的再次筛
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