Question

我一直在寻找答案，我发现了CASE，IFF和UNION，但我还没有弄清楚，下面是我一直在尝试的。 / p>

SELECT *
    FROM car_label as a
    INNER JOIN car as b
    ON a.label_id = b.label_id
    WHERE a.car_id = 619
UNION ALL
SELECT *
    FROM car
    WHERE car_id = 619

如果第一个选择返回空，我想改为运行第二个选择。第一部分完美地运作，SELECT * FROM car_label as a INNER JOIN car as b ON a.label_id = b.label_id WHERE a.car_id = :car_id，我只是试图包含第二个选择，以防第一个选择为空。

更新

Message: 'SQLSTATE[42000]: Syntax error or access violation: 1064 You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'IF (SELECT * FROM car_label as a INNER JOIN car as' at line 1'

Answer 1

试试这个：

SELECT *
FROM car_label as a
INNER JOIN car as b ON a.label_id = b.label_id
WHERE a.car_id = 619
UNION ALL
SELECT *
FROM car
WHERE car_id = 619
  AND NOT EXISTS (SELECT 1 
                  FROM car_label as a
                  INNER JOIN car as b ON a.label_id = b.label_id
                  WHERE a.car_id = 619)

因此，如果第一个查询返回任何数据，则NOT EXISTS将为真，因此第二个查询条件为真，反之亦然。

请注意，为了使此查询起作用，返回的列应具有相同的计数和相同的数据类型，因此您可能需要列出列而不是*。

然而，我认为您可能需要尝试LEFT JOIN所以如果car_label中没有匹配的汽车，您仍会从car表中获取数据：

SELECT b.*
FROM car as b
LEFT JOIN car_label as a ON a.label_id = b.label_id AND a.car_id = 619

Answer 2

我不太确定你的查询。据我了解，你可以去IF EXISTS

尝试下面

MSSQL

 IF EXISTS(SELECT * FROM car_label as a INNER JOIN car as b ON a.label_id = b.label_id WHERE a.car_id = 619) 
 BEGIN 
        SELECT *
        FROM car_label as a INNER JOIN car as b ON a.label_id = b.label_id
        WHERE a.car_id = 619
 END
 ELSE
 BEGIN
        SELECT *
        FROM car
        WHERE car_id = 619
 END

<强>的MySQL

 IF (SELECT * FROM car_label as a INNER JOIN car as b ON a.label_id = b.label_id WHERE a.car_id = 619) THEN
 BEGIN 
        SELECT *
        FROM car_label as a INNER JOIN car as b ON a.label_id = b.label_id
        WHERE a.car_id = 619
 END;
 ELSE
 BEGIN
        SELECT *
        FROM car
        WHERE car_id = 619
 END;
 END IF;

如果第一个是空的，如何运行第二个SQL选择？

2 个答案: