mysql - 选择数据时mysql查询中的逻辑错误

Question

表pc -

 code   model   speed   ram hd  cd  price
 1  1232    500 64  5.0 12x 600.0000
 10 1260    500 32  10.0    12x 350.0000
 11 1233    900 128 40.0    40x 980.0000
 12 1233    800 128 20.0    50x 970.0000
 2  1121    750 128 14.0    40x 850.0000
 3  1233    500 64  5.0 12x 600.0000
 4  1121    600 128 14.0    40x 850.0000
 5  1121    600 128 8.0 40x 850.0000
 6  1233    750 128 20.0    50x 950.0000
 7  1232    500 32  10.0    12x 400.0000
 8  1232    450 64  8.0 24x 350.0000
 9  1232    450 32  10.0    24x 350.0000

期望的输出-

model   speed   hd
1232    450 10.0
1232    450 8.0
1232    500 10.0
1260    500 10.0

查询 1 -

SELECT model, speed, hd
FROM pc
WHERE cd = '12x' AND price < 600
OR
cd = '24x' AND price < 600

查询 2 -

SELECT model, speed, hd
FROM pc
WHERE cd = '12x' OR cd = '24x' 
AND price < 600

查询 1 确实工作正常，但是当我尝试将查询减少为仅一次使用价格时，它没有显示正确的结果..让我知道我在逻辑中遗漏了什么。

Find the model number, speed and hard drive capacity of the PCs having 12x CD and prices less than $600 or having 24x CD and prices less than $600.

Answer 1

由于 AND 出现在 OR 之前，您的查询被解释为:

WHERE (cd = '12x') OR ((cd = '24x') AND (price < 600))

或者，换句话说:所有具有 12x CD 的 PC，或具有 24x CD 的 PC < 600 美元

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您需要使用括号来指定操作顺序:

WHERE (cd = '12x' OR cd = '24x') AND price < 600

或者，您可以使用 IN:

WHERE cd IN ('12x', '24x') AND price < 600

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另请参阅:http://dev.mysql.com/doc/refman/5.5/en/operator-precedence.html