SQL - 仅当子查询不为空时更新

Question

只有当 car_log 中有没有状态 'test' 的条目时我才想更新 last_test_date 我真的认为这段代码很糟糕，有什么办法可以更好地编写它？

UPDATE car SET last_test_date = 
(SELECT date FROM car_log WHERE car_log.car_id = car.id AND
car_log.status != 'test' ORDER BY date DESC LIMIT 0,1)
WHERE (SELECT COUNT(*) FROM car_log WHERE car_log_id = client.id AND
car_log.status != 'test') > 0;

Answer 1

使用@@ROWCOUNT获取最后查询的行数，这样就不会重复select

...
WHERE @@ROWCOUNT > 0;

Answer 2

更新连接语法将使您的语句更加清晰：

UPDATE car 
JOIN   (SELECT car_id, MAX(date) AS max_date
        FROM   car_log
        WHERE  car_log.status != 'test') ON car_log.car_id = car.id AND
SET    last_test_date = max_date

SQL - 仅当子查询不为空时更新

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