What are the benefits of using mysql's case method?

The benefits of using the case method of mysql: 1. Flexible organization format when displaying query results; 2. Effectively avoiding multiple accesses to the same table or several tables.

Benefits of using mysql's case method:

select has two biggest benefits when used in conjunction with case. First, the format can be flexibly organized when displaying query results, and second, multiple accesses to the same table or several tables are effectively avoided.

The following is a simple example to illustrate. For example, the table students(id, name, birthday, sex, grade) requires statistics on the number of boys and girls according to each grade.

The header of the statistical results is, grade, number of boys, number of girls . If you don't use select case when, in order to display the number of men and women side by side, it will be very troublesome to count. You must first determine the grade information, and then take the number of boys and girls according to the grade, and it is easy to make mistakes.

Use select case when to write as follows:

SELECT grade, COUNT (CASE WHEN sex = 1 THEN 1     
                            ELSE NULL
                        END) 男生数,
               COUNT (CASE WHEN sex = 2 THEN 1
                            ELSE NULL
                       END) 女生数
FROM students
GROUP BY grade;

More related free learning recommendations: mysql tutorial (video)

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