Question

所以我想根据一个字段是否为空来运行不同的查询。这就是我所拥有的

我得到的错误是SELECT CASE WHEN vehicle_uid IS NULL THEN (SELECT lic_num, vehicle_make, vehicle_model, zone.name FROM notice INNER JOIN zone on notice.zone_uid = zone.uid WHERE lic_num IN ( SELECT lic_num FROM notice GROUP BY lic_num HAVING count(*) > 1 )) ELSE (SELECT lic_num, vehicle_uid, model.model, make.make, zone.name FROM notice INNER JOIN vehicle_model AS model ON notice.vehicle_uid = model.uid INNER JOIN vehicle_make AS make ON model.vehicle_make_uid = make.uid INNER JOIN zone on notice.zone_uid = zone.uid WHERE lic_num IN ( SELECT lic_num FROM notice GROUP BY lic_num HAVING count(*) > 1 )) END当我将#1054 - Unknown column 'vehicle_uid' in 'field list'更改为WHEN vehicle_uid IS NULL时，错误显示为：WHEN notice.vehicle_uid IS NULL

我也试过了 #1109 - Unknown table 'notice' in field list出现以下错误：SELECT CASE vehicle_uid WHEN IS NULL THEN ...

我只是从更复杂的查询开始，并且不知道为什么这是错误的。

Answer 1

您在外部查询中缺少FROM子句。

Answer 2

您不能在select语句中执行子查询，该语句返回多个字段，也不会返回多行。您看到的错误是因为您从无处选择了一个字段vehicle_uid，您不会在数据库中找到from子句，知道它来自哪里。

解决问题的最佳方法是使用特定条件执行UNION，但仅当两个查询具有相同的列集时才会起作用：

SELECT lic_num, 
       vehicle_make,
       '' SUBSTITUTE_FORVEHICLEUID,
       vehicle_model, 
       '' SUBSTITUTE_FOR_MAKE,
       zone.name
  FROM notice
        INNER JOIN zone ON notice.zone_uid = zone.uid
 WHERE lic_num IN (
           SELECT lic_num 
             FROM notice
            GROUP BY lic_num 
           HAVING count(*) > 1
         )
   AND vehicle_uid is null
UNION
SELECT lic_num,  
       '' SUBSTITUTE_FOR_VEHICLEMAKE, 
       vehicle_uid, 
       model.model, 
       make.make, 
       zone.name
  FROM notice
      INNER JOIN vehicle_model AS model ON notice.vehicle_uid = model.uid
      INNER JOIN vehicle_make AS make ON model.vehicle_make_uid = make.uid
      INNER JOIN zone on notice.zone_uid = zone.uid
 WHERE lic_num IN (
        SELECT lic_num 
        FROM notice
        GROUP BY lic_num 
        HAVING count(*) > 1
      )
   AND vehicle_uid IS NOT NULL

即使这是您问题的最佳选择，您也必须调整两个查询。因为我不知道你的结构是我能帮到的最好的结构。

Answer 3

您应该将UNION与两个返回相同列的SELECT语句一起使用。

(SELECT lic_num, vehicle_make uid_or_make, vehicle_model, '', zone.name
    FROM notice
    INNER JOIN zone on notice.zone_uid = zone.uid
    WHERE lic_num IN (SELECT lic_num FROM notice GROUP BY lic_num  HAVING count(*) > 1) AND vehicle_uid IS NULL
)
UNION ALL
(SELECT lic_num, vehicle_uid uid_or_make, model.model vehicle_model, make.make, zone.name
    FROM notice
    INNER JOIN vehicle_model AS model ON notice.vehicle_uid = model.uid
    INNER JOIN vehicle_make AS make ON model.vehicle_make_uid = make.uid
    INNER JOIN zone on notice.zone_uid = zone.uid
    WHERE lic_num IN (SELECT lic_num FROM notice GROUP BY lic_num HAVING count(*) > 1) AND vehicle_uid IS NOT NULL
)

如果field为null，则运行特定查询

3 个答案: