case when 字段1 in (值1，值2，值3) then 字段2 end中括号的内容如何自动获取值？
select case when 字段1 in (值1，值2，值3) then 字段2 end where 字段1 in (值1，值2，值3) group by 字段1，字段2上式有没有取巧的办法，省掉其中…
多重条件嵌套查询语句的优化？
SELECT * FROM table1 t1 WHERE t1.pk <> (SELECT m1 FROM table2@db1 WHERE m2= (SELECT t3.pk FROM table3@db2 t3 WHERE t1.pk=t3.pk))…
多重条件嵌套的查询语句如何优化？
SELECT * FROM table1 t1 WHERE t1.m <> (SELECT m FROM table2@db1 WHERE m2= (SELECT t3.pk FROM table3@db2 t3 WHERE t1.pk=t3.pk))表1和表3有…
为什么group by 和where在count函数的表现一个为空一个就可以得0？
句1:select count(主键) from 表1 group by 主键 having count(*)>1 结果为null句2:select count(主键) from 表1 where instr(主键,'空格')<>0 …
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