How does MySQL convert rows to columns?

I don't say much nonsense , Let's look directly at an example ：

This is a student transcript we have built student_score, Let's take a look at the results after the conversion , Here's the picture ：

  It can be seen that , Here, the row to column is to change the original subject Select multiple lines of the field , As different columns in the result set , According to userid Group and display the corresponding score, We generally have two ways to realize

1、 Use  CASE...WHEN...THEN  sentence

SELECT userid,
       SUM(CASE `subject` WHEN ' Chinese language and literature ' THEN score ELSE 0 END) as ' Chinese language and literature ',
       SUM(CASE `subject` WHEN ' mathematics ' THEN score ELSE 0 END) as ' mathematics ',
       SUM(CASE `subject` WHEN ' English ' THEN score ELSE 0 END) as ' English ',
       SUM(CASE `subject` WHEN ' Politics ' THEN score ELSE 0 END) as ' Politics '  
FROM student_score
GROUP BY userid

for example CASE `subject` WHEN ' Chinese language and literature ’ THEN score ELSE 0 END This statement , That is, for all fields subject=` Chinese language and literature ` Conduct SUM() operation , Set the value to score, If not, set to 0

2、 Use IF() function

SELECT userid,
       SUM(IF(`subject`=' Chinese language and literature ',score,0)) as ' Chinese language and literature ',
       SUM(IF(`subject`=' mathematics ',score,0)) as ' mathematics ',
       SUM(IF(`subject`=' English ',score,0)) as ' English ',
       SUM(IF(`subject`=' Politics ',score,0)) as ' Politics '
FROM student_score 
GROUP BY userid

IF(subject=` Chinese language and literature `,score,0) As a condition , For all subject=` Chinese language and literature ` The record of score The fields go SUM() operation , If score If there is no value, the default value is 0.

Be careful :SUM() To be able to use GROUP BY according to userid Grouping , Because every one of them userid Corresponding subject=" Chinese language and literature " There's only one record of , therefore SUM() The value of is equal to that of the corresponding record score Value . If userid ='001' and subject=' Chinese language and literature ' There are two records of , Now SUM() The value of will be the sum of these two records , Empathy , Use Max() The value of will be the largest of the two records . But normally , One user Corresponding to one subject There's only one score , So you can use SUM()、MAX()、MIN()、AVG() Aggregate function can achieve the effect of row to column .