mySql关于统计数量的SQL查询操作

我就废话不多说了，大家还是直接看代码吧~

									select project_no,

									sum(case when device_state=0 then 1 else 0 end)as offTotal ,

									sum(case when device_state=1 then 1 else 0 end)as onlineTotal,

									sum(1)total

									from iot_d_device

									group by project_no

									order by project_no

补充：MySQL一条SQL语句查询多条统计结果

商城项目难免会遇到用户个人中心页查询不同状态订单数量的问题。当然这个问题并不难，可以写一个DAO层方法，以状态作为入参，每次传入不同状态值依次查询相应状态的订单数量。

今天在写H5端接口时，我想换种方式查，也就是通过一条SQL查询出多个状态的订单数量。在网上搜了搜，方法可行，所以就尝试了下，果不其然成功了。

示例如下（数据只为演示今天的问题，表设计并不严谨。勿怪）：

									SET FOREIGN_KEY_CHECKS=0;

									-- ----------------------------

									-- Table structure for mini_test_order

									-- ----------------------------

									DROP TABLE IF EXISTS `mini_test_order`;

									CREATE TABLE `mini_test_order` (

									 `id` int(11) NOT NULL,

									 `order_no` varchar(32) DEFAULT NULL COMMENT '订单号',

									 `user_id` int(11) DEFAULT NULL COMMENT '用户id',

									 `shop_id` int(11) DEFAULT NULL COMMENT '商家id',

									 `order_status` tinyint(1) DEFAULT NULL COMMENT '订单状态',

									 `create_time` int(10) DEFAULT NULL COMMENT '创建时间',

									 PRIMARY KEY (`id`)

									) ENGINE=InnoDB DEFAULT CHARSET=utf8;

									-- ----------------------------

									-- Records of mini_test_order

									-- ----------------------------

									INSERT INTO `mini_test_order` VALUES ('1', 'aaaaaaaaa', '11', '111', '1', '1573041313');

									INSERT INTO `mini_test_order` VALUES ('2', 'bbbbbbbb', '11', '222', '1', '1573041313');

									INSERT INTO `mini_test_order` VALUES ('3', 'cccccccccc', '11', '333', '2', '1573041313');

									INSERT INTO `mini_test_order` VALUES ('4', 'dddddddd', '11', '222', '3', '1573041313');

									INSERT INTO `mini_test_order` VALUES ('5', 'eeeeeeeee', '11', '111', '4', '1573041313');

									INSERT INTO `mini_test_order` VALUES ('6', 'ffffffffffffff', '11', '111', '3', '1573041313');

									INSERT INTO `mini_test_order` VALUES ('7', 'gggggggg', '11', '222', '4', '1573041313');

									INSERT INTO `mini_test_order` VALUES ('8', 'hhhhhhhhh', '11', '111', '4', '1573041313');

									INSERT INTO `mini_test_order` VALUES ('9', 'iiiiiiiiiiiiiiiiiii', '11', '333', '3', '1573041313');

									INSERT INTO `mini_test_order` VALUES ('10', 'jjjjjjjjjjjjjjjjjj', '11', '222', '1', '1573041313');

核心SQL语句如下：

SELECT COUNT(CASE order_status WHEN 1 THEN 1 END) AS "状态1",COUNT(CASE order_status WHEN 2 THEN 1 END) AS "状态2",COUNT(CASE order_status WHEN 3 THEN 1 END) AS "状态3",COUNT(CASE order_status WHEN 4 THEN 1 END) AS "状态4" FROM `mini_test_order`;

或如下：

SELECT COUNT(CASE WHEN order_status = 1 THEN 1 END) AS "状态1",COUNT(CASE WHEN order_status = 2 THEN 1 END) AS "状态2",COUNT(CASE WHEN order_status = 3 THEN 1 END) AS "状态3",COUNT(CASE WHEN order_status = 4 THEN 1 END) AS "状态4" FROM `mini_test_order` ;

当然，SQL语句不仅仅局限于上述两种写法，喜欢探究的童靴欢迎留言补充。

MySQL的case when的语法有两种

1.简单函数