mysql 如何判断大小而 更新 字段

$sql="update `a` set `open`=if(`open`<$d,$d,`open`),`close`=if(`close`>$d,$d,`close`)"

这样应该行吧，不过where条件没加哦

最大值：select max(num) from table

第二大值：select max(num) from table

where num not in(select max(num) from table)

第三大值：select max(num) from table

where num not in(select max(num) from table

where num not in(select max(num) from table))

最小值：select min(num) from table

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