MySQL问题：带有子查询的内部联接没有返回结果

Question

我有一个数据库，其中包含一组具有不同表单版本版本的表单。 下面的“有效”查询按每个表单版本的批准使用日期成功选择了最新的表单版本。

我想将此查询“工作”查询与另一个表“ forms_types”结合起来，该表返回“ form_type”列的全文描述。

我这样做的尝试不起作用-我也应该在查询的“联接”版本上获得50个结果，但它不返回任何结果。

让我的工作查询正确加入，将不胜感激！

工作（50个结果，但没有JOIN）：

SELECT *
FROM forms AS t
WHERE california =
    ( 
    SELECT MAX(california)
    FROM forms
    WHERE `form_number` = t.`form_number`
    AND trucking_coverage= 1
    AND california IS NOT NULL
    AND california <= '2014-01-29'
    AND form_type <> 0
    )
ORDER BY `t`.`form_type` ASC, `t`.`form_number` ASC

不起作用（无结果）：

SELECT *
FROM forms AS t
INNER JOIN forms_types 
ON 'forms.form_type' = 'forms_types.form_type'
WHERE california =
    ( 
    SELECT MAX(california)
    FROM forms
    WHERE `form_number` = t.`form_number`
    AND trucking_coverage= 1
    AND california IS NOT NULL
    AND california <= '2014-01-29'
    AND form_type <> 0
    )
ORDER BY `t`.`form_type` ASC, `t`.`form_number` ASC

Answer 1

您是否正在寻找这样的东西？

SELECT f.*, t.*
  FROM 
( 
  SELECT form_number, MAX(california) california
    FROM forms
   WHERE trucking_coverage = 1
     AND california IS NOT NULL
     AND california <= '2014-01-29'
     AND form_type <> 0
   GROUP BY form_number
) q JOIN forms f
    ON q.form_number = f.form_number
   AND q.california = f.california JOIN forms_types t
    ON f.form_type = t.form_type
 ORDER BY f.form_type, f.form_number

也可以用这种方式重写（通过利用非标准的MySQL GROUP BY扩展名）

SELECT *
  FROM
(
  SELECT *
    FROM forms f JOIN forms_types t
      ON f.form_type = t.form_type
   WHERE f.trucking_coverage = 1
     AND f.california IS NOT NULL
     AND f.california <= '2014-01-29'
     AND f.form_type <> 0
   ORDER BY california DESC
) q
 GROUP BY f.form_number
 ORDER BY f.form_type, f.form_number