Java – MySQL-Abfrage, wie man das abfragt

Question

Ich habe eine Interviewfrage gesehen, die von jemand anderem in der Gruppe gepostet wurde, =. =, wie frage ich ab?

Answer 1

题图是Chinase,按这个来。
Course确定情况下：
CASE实现：

SELECT
    `Name`,
    MAX(
        CASE
        WHEN Course = 'Chinase' THEN
            Score
        END
    ) AS Chinase,
    MAX(
        CASE
        WHEN Course = 'Math' THEN
            Score
        END
    ) AS Math,
    MAX(
        CASE
        WHEN Course = 'English' THEN
            Score
        END
    ) AS English
FROM
    table1
GROUP BY
    `Name`

IF实现：

SELECT
    `Name`,

SUM(IF (Course = 'Chinase', Score, 0)) AS Chinase,

SUM(IF (Course = 'Math', Score, 0)) AS Math,

SUM(IF (Course = 'English', Score, 0)) AS English

FROM
    table1
GROUP BY `Name`

Course不确定，使用动态：

SET @CC='';
SELECT @CC:=CONCAT(@CC,'SUM(IF(Course=\'',Course,'\'',',Score,0)) AS ',Course,',') FROM (SELECT DISTINCT Course FROM table1) A;
SET @HH=CONCAT('SELECT Name,',LEFT(@CC,LENGTH(@CC)-1),' FROM table1 GROUP BY Name');
PREPARE stmt FROM @HH;
EXECUTE stmt;

Answer 2

select name, sum(case when Course='Chinese' then Score end) as Chinese,
sum(case when type='Math' then Score end) as Math,
sum(case when type='English' then Score end) as English
from table1
group by name

Answer 3

你应该缺学习一下原理
百度"mysql行列转换"
只学会这一个sql也没什么用

Answer 4

做数据统计的时候会用到这种神一样的sql，平时phper工作上是用不上的。