SQL-仅在子查询不为null时更新

Question

我只想在car_log中有没有状态“ test”的条目时更新last_test_date，我真的认为这段代码太可怕了，有什么办法可以更好地编写它吗？

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获取最后一个查询行数，这样就不会重复选择

...
WHERE @@ROWCOUNT > 0;

Answer 2

更新联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