在SQL中添加括号会产生不同的结果

Question

我创建了两个SQL语句，第二个给出的结果要少得多。唯一的区别是我在第二个查询中添加了额外的括号：

SELECT COUNT(*)
FROM records
JOIN other_records ON records.other_record_id = other_records.id
WHERE other_records.practice = 'Medical' && 
records.details IS NULL || 
(records.details <> 'ERROR' && records.details <> 'ERROR BY PRACTICE');

+----------+
| COUNT(*) |
+----------+
|   342668 |
+----------+
1 row in set (3.42 sec)

SELECT COUNT(*) FROM records JOIN other_records 
ON records.other_record_id = other_records.id 
WHERE other_records.practice = 'Medical' && 
(records.details IS NULL || (records.details <> 'ERROR' && records.details <> 'ERROR BY PRACTICE'));

+----------+
| COUNT(*) |
+----------+
|   193899 |
+----------+
1 row in set (0.46 sec)

据我了解，首先它将JOIN的结果与other_records.practice匹配。两者都应返回相同的结果集。然后，据我所知，它将匹配细节为null或细节与ERROR和ERROR BY PRACTICE不匹配的地方。我不确定为什么括号会在这里有所作为？

Answer 1

在布尔逻辑中，和运算符&&优先于或运算符||。

因此，当您编写此代码时：

WHERE other_records.practice = 'Medical' && 
records.details IS NULL ||
(records.details <> 'ERROR' && records.details <> 'ERROR BY PRACTICE')

...等效于此：

WHERE (other_records.practice = 'Medical' && records.details IS NULL)
||
(records.details <> 'ERROR' && records.details <> 'ERROR BY PRACTICE')

...因此将不同的结果添加到第二条语句中。