Question

是否可以在SELECT之前使用SELECT而不使用SET？

不知怎的'@period'变量并不总是正常工作。例如，在MySQL Workbench（v5.2）中，结果仅在第二次调用后显示，如果更改了变量，结果将仅在第二次运行时才正确。 phpMyAdmin显示正确的行数，但没有条目，有人可以解释一下使用变量这样的错误：

    select sql_no_cache @num := @num + 1 as `#`, l.* from (

    select s.* from (
    select
    @num := 0 as 'Label 1',
    @period := 31 as 'LAbel 2',
    '' as 'Label 3'

    union

    select concat(users.first_name, ' ',users.last_name),u.type, u.date, u.description from (

    select c.user_id as uguid, 'type1' as type, c.date_start as date, c.description from table1 c 
    where deleted = 0
    and date_start > SUBDATE(CURRENT_DATE,@period)

    union
    ...
) as u) as l

Answer 1

发现了一个错误。原因是变量在定义之前使用。 '@period'在查询中定义，但它首先在首先执行的子查询中出现。因此，解决方案是将变量定义与其使用的子查询放在同一级别上。

类似的东西：

    select sql_no_cache @num := @num + 1 as `#`, l.* from (

        select s.* from (
        select
        @num := 0 as 'Label 1',
        '' as 'LAbel 2',
        '' as 'Label 3'

        union

        select concat(users.first_name, ' ',users.last_name),u.type, u.date, u.description from     (
            select o2.* from
             (select @period:=31) as o1
             join (
               select c.user_id as uguid, 'type1' as type, c.date_start as date, c.description from table1 c 
               where deleted = 0
                 and date_start > SUBDATE(CURRENT_DATE,@period)

              union
                ...
             ) as o2
       ) as u
   ) as l

MySQL在SELECT子查询中使用变量

1 个答案: