MySQL column to row conversion without Union

Recently, I have used mysql Column turned , I found a lot of methods on the Internet, most of which are row to column methods , For column to row , Only found union This way , However, the database version of the working environment is low and does not support temporary tables , Use union The way of writing is too cumbersome , So here is a record of the use union And don't use union Column to row solution

notes ： The execution efficiency of the two methods is not compared

Test data ：

CREATE TABLE `test` (
  `id` int NOT NULL AUTO_INCREMENT,
  `name` varchar(255) DEFAULT NULL,
  `chinese` int DEFAULT NULL,
  `math` int DEFAULT NULL,
  `english` int DEFAULT NULL,
  `wuli` int DEFAULT NULL,
  `huaxue` int DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;

INSERT INTO test.test (id, name, chinese, math, english, wuli, huaxue) VALUES(1, ' Zhang San ', 110, 120, 85, NULL, NULL);
INSERT INTO test.test (id, name, chinese, math, english, wuli, huaxue) VALUES(2, ' Li Si ', 130, 88, 89, NULL, NULL);
INSERT INTO test.test (id, name, chinese, math, english, wuli, huaxue) VALUES(3, ' Wang Wu ', 93, 124, 87, 98, 67);

Basic union Column to row writing ：

--  First use union Implement column to row 
select name,' Chinese language and literature ' as course, chinese as 'score' from test union 
select name,' mathematics ' as course, math as 'score' from test union 
select name,' English ' as course, english as 'score' from test union 
select name,' Physics ' as course, wuli as 'score' from test union 
select name,' chemical ' as course, huaxue as 'score' from test order by name asc

Realization effect ：

Don't use union Column to row writing ：

no need union The way to realize the idea ：

The main core of column to row conversion is to convert one data to multiple data , If you do not query many times , You want to convert columns to rows , We need to expand the number of pieces of data according to our query requirements ; At the same time, in order to ensure that the expanded results can meet the needs of our subsequent data extraction , Therefore, it is necessary to fill the field name into the data for field extraction while expanding .

--  With the help of system tables （ Or construct temporary data by yourself ） use join  Amplification data , And then use case when  Implement column to row , Get the desired result set .
select  
	b.name,
	a.COLUMN_NAME as  subject ,
	case a.COLUMN_NAME when 'chinese' then b.chinese when 'math' then b.math
	     when 'english' then b.english when 'wuli' then b.wuli  when 'huaxue' then b.huaxue end as  achievement 
from 
--  Get the field list with the help of the system table , Add custom joinid Used for data association 
	(select  COLUMN_NAME ,'aa' as joinid from  information_schema.`COLUMNS` c  where TABLE_NAME ='test' and column_name not in ('id','name')) a 
--  Use customization joinid join  Get Cartesian product result set 
	left  join 
--  Query source table , Add custom joinid Used for data association 
	(select *,'aa' as joinid from test t ) b 
	on a.joinid=b.joinid order by b.name

Process demonstration ：

Get the field name from the system table , At the same time, the user-defined Association column is added ：

Use join Associated field name table and data table , Complete data construction ：

Use case when Extract the desired data , Achieve the goal of column to row ：