在看到一位同事写的sql语句,发现,在同一张表统计不同条件的数量的个数时是这样写的(公司数据库表就不贴了,新建了一张简单表记录下)
表结构如下:
DROP TABLE IF EXISTS `al_users`;
CREATE TABLE `al_users` (
`id` bigint(20) NOT NULL AUTO_INCREMENT,
`pid` int(11) DEFAULT NULL,
`username` varchar(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci DEFAULT NULL,
`password` varchar(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci DEFAULT NULL,
`age` int(11) DEFAULT NULL,
`is_del` tinyint(1) DEFAULT NULL,
PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB AUTO_INCREMENT = 4 CHARACTER SET = utf8mb4 COLLATE = utf8mb4_general_ci ROW_FORMAT = Dynamic;
-- ----------------------------
-- Records of al_users
-- ----------------------------
INSERT INTO `al_users` VALUES (1, 0, 'zhangsan', 'zhangsan', 10, 0);
INSERT INTO `al_users` VALUES (2, 0, 'lisi', 'lisi', 21, 0);
INSERT INTO `al_users` VALUES (3, NULL, 'wangwu', 'wangwu', 26, 1);
SET FOREIGN_KEY_CHECKS = 1;
现需求如下:统计同一个父级id下的总用户数,和未删除的用户数
同事试这样写的:
select
(select count(id) FROM al_users) AS total,
(select count(id) FROM al_users WHERE is_del = 0) AS unDelTotal
FROM al_users
GROUP BY al_users.pid
优化后:
select count(id) AS total,
count(is_del=0) AS unDelTotal
FROM al_users
GROUP BY al_users.pid
这几条数据测试时运行时间还看不出来太大的差别,数据量大的话,差别就出来了。
后记:
mysql中的sum函数,当参数是具体列名时,是求和,当参数是条件时,是统计条数