MySQL associative table queries (reducing the number of queries)

Mysql Unrelated linked table queries （ Reduce the number of queries ）

Preface

In order to reduce the number of queries to the database , For example, in unrelated tables, in order to reduce the pressure of the system , We can go through union all Keyword perform an associated query on the data found in multiple tables
（ It is convenient to use different data for statistical analysis and only use one request ）

give an example ： Pass one sql Statement query the total number of students whose gender is male in the student table and the total number of teachers whose gender is male in the teacher table at one time


Database table preparation ：
1、student surface

SET NAMES utf8mb4;
SET FOREIGN_KEY_CHECKS = 0;

-- ----------------------------
-- Table structure for student
-- ----------------------------
DROP TABLE IF EXISTS `student`;
CREATE TABLE `student`  (
  `id` varchar(20) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci NOT NULL,
  `name` varchar(20) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci NOT NULL DEFAULT '',
  `birth` varchar(20) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci NOT NULL DEFAULT '',
  `sex` varchar(10) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci NOT NULL DEFAULT '',
  PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB CHARACTER SET = utf8mb4 COLLATE = utf8mb4_0900_ai_ci ROW_FORMAT = Dynamic;

-- ----------------------------
-- Records of student
-- ----------------------------
INSERT INTO `student` VALUES ('01', ' Zhao Lei ', '1990-01-01', ' male ');
INSERT INTO `student` VALUES ('02', ' Qian Dian ', '1990-12-21', ' male ');
INSERT INTO `student` VALUES ('03', ' Sun Feng ', '1990-05-20', ' male ');
INSERT INTO `student` VALUES ('04', ' Li Yun ', '1990-08-06', ' male ');
INSERT INTO `student` VALUES ('05', ' Zhou Mei ', '1991-12-01', ' Woman ');
INSERT INTO `student` VALUES ('06', ' Wu Lan ', '1992-03-01', ' Woman ');
INSERT INTO `student` VALUES ('07', ' Zheng Zhu ', '1989-07-01', ' Woman ');
INSERT INTO `student` VALUES ('08', ' Wangju ', '1990-01-20', ' Woman ');

SET FOREIGN_KEY_CHECKS = 1;

2、teacher surface

SET NAMES utf8mb4;
SET FOREIGN_KEY_CHECKS = 0;

-- ----------------------------
-- Table structure for teacher
-- ----------------------------
DROP TABLE IF EXISTS `teacher`;
CREATE TABLE `teacher`  (
  `id` varchar(20) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci NOT NULL,
  `name` varchar(20) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci NOT NULL DEFAULT '',
  `sex` varchar(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci NULL DEFAULT NULL,
  PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB CHARACTER SET = utf8mb4 COLLATE = utf8mb4_0900_ai_ci ROW_FORMAT = Dynamic;

-- ----------------------------
-- Records of teacher
-- ----------------------------
INSERT INTO `teacher` VALUES ('01', ' Zhang San ', ' male ');
INSERT INTO `teacher` VALUES ('02', ' Li Si ', ' Woman ');
INSERT INTO `teacher` VALUES ('03', ' Wang Wu ', ' male ');

SET FOREIGN_KEY_CHECKS = 1;

One 、 traditional method （ Poor query performance ）

Encapsulate the queried data into a table , It is displayed in the data query of the table respectively .
This method is relatively simple, but it will greatly improve the number of queries to the database

SELECT
	t1. Total number of male students ,
	t2. Total number of male teachers  
FROM
	( SELECT count( id ) AS  Total number of male students  FROM student WHERE student.sex = ' male ' ) t1,
	( SELECT count( id ) AS  Total number of male teachers  FROM teacher WHERE teacher.sex = ' male ' ) t2

Two 、 Use union all Combine multiple tables into a table query

	select t.*  from
	
	( SELECT count(id) as a,0 as b FROM student WHERE student.sex = ' male '
			union all
	SELECT 0 as a,count(id) as b FROM teacher WHERE teacher.sex = ' male ' ) t

1、 here a Represents the total number of students whose gender is male ,b Represents the total number of teachers whose gender is male

2、 At this point, we just need to a and b Sum separately , You can find out the total number of male students and male teachers

	select sum(t.a) as  Total number of male students ,sum(t.b) as  Total number of male teachers  from
	( SELECT count(id) as a,0 as b FROM student WHERE student.sex = ' male '
			union all
	SELECT 0 as a,count(id) as b FROM teacher WHERE teacher.sex = ' male ' ) t

3、 ... and 、 summary

When we use union all When multiple tables are combined , In this way, for unrelated data, we can only query multiple pieces of data we need at a time , Reduce the number of requests and queries , And the performance of the database is greatly improved ！