這個(gè)可能是容易被忽略的問題，首選我們要清楚：MySQL中，AND的執(zhí)行優(yōu)先級(jí)高于OR。也就是說，在沒有小括號(hào)()的限制下，總是優(yōu)先執(zhí)行AND語(yǔ)句，再執(zhí)行OR語(yǔ)句。比如：

select * from table where 條件1 AND 條件2 OR 條件3等價(jià)于select * from table where ( 條件1 AND 條件2 ) OR 條件3select * from table where 條件1 AND 條件2 OR 條件3 AND 條件4等價(jià)于select * from table where ( 條件1 AND 條件2 ) OR ( 條件3 AND 條件4 )

來點(diǎn)事例深入理解下：

測(cè)試表數(shù)據(jù)：

SET NAMES utf8mb4;SET FOREIGN_KEY_CHECKS = 0;-- ------------------------------ Table structure for book-- ----------------------------DROP TABLE IF EXISTS `book`;CREATE TABLE `book` ( `id` int(10) UNSIGNED NOT NULL AUTO_INCREMENT, `name` varchar(25) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci DEFAULT NULL, `author` varchar(25) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci DEFAULT NULL, `price` decimal(10, 2) DEFAULT NULL, PRIMARY KEY (`id`) USING BTREE) ENGINE = InnoDB CHARACTER SET = utf8mb4 COLLATE = utf8mb4_0900_ai_ci ROW_FORMAT = Dynamic;-- ------------------------------ Records of book-- ----------------------------INSERT INTO `book` VALUES (1, ’PHP’, ’mate’, 21.00);INSERT INTO `book` VALUES (2, ’JAVA’, ’kaven’, 23.00);INSERT INTO `book` VALUES (3, ’JAVA高級(jí)’, ’loose’, 45.00);INSERT INTO `book` VALUES (4, ’GO’, ’jim’, 46.00);INSERT INTO `book` VALUES (5, ’GO設(shè)計(jì)’, ’json’, 76.00);INSERT INTO `book` VALUES (6, ’PHP高級(jí)編程’, ’bate’, 67.00);INSERT INTO `book` VALUES (7, ’Python’, ’jim’, 66.00);INSERT INTO `book` VALUES (8, ’Python設(shè)計(jì)’, ’mali’, 54.00);INSERT INTO `book` VALUES (9, ’GO編程’, ’kaven’, 86.00);INSERT INTO `book` VALUES (11, ’Python3’, ’jim’, 55.00);SET FOREIGN_KEY_CHECKS = 1;

查詢方式1：

SELECT * FROM book WHERE author=’jim’ OR author=’json’ AND name=’PHP’;

上面的查詢等價(jià)于：

SELECT * FROM book WHERE author=’jim’ OR (author=’json’ AND name=’PHP’);

那么上面的查詢結(jié)果就很好理解了。

查詢方式2：

SELECT * FROM book WHERE name=’PHP’ AND author=’jim’ OR author=’json’;

上面的查詢等價(jià)于：

SELECT * FROM book WHERE (name=’PHP’ AND author=’jim’) OR author=’json’;

查詢方式3：

SELECT * FROM book WHERE name=’GO’ AND (author=’jim’ OR author=’json’);

這個(gè)就很好理解了。了解and or的優(yōu)先級(jí)。這些查詢也就不是呢么'理解混淆'了。

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