【躍遷之路】【421天】刻意練習系列180—SQL（2018.04.02）

@(躍遷之路)專欄

叨叨兩句

1. 技術的精進不能只是簡單的刷題，而應該是不斷的「刻意」練習
2. 該系列改版後正式歸入【躍遷之路】專欄，持續更新

刻意練習——MySQL

2018.04.02

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題目描述

DROP TABLE IF EXISTS test1;
CREATE TABLE test1 (
id int(11) NOT NULL AUTO_INCREMENT,
username varchar(20) NOT NULL,
course varchar(20) NOT NULL,
score bigint(20) NOT NULL,
PRIMARY KEY (id)
) ENGINE=InnoDB AUTO_INCREMENT=10 DEFAULT CHARSET=utf8;

INSERT INTO test1 VALUES ('1', '張三', '數學', '34');
INSERT INTO test1 VALUES ('2', '張三', '語文', '44');
INSERT INTO test1 VALUES ('3', '張三', '英語', '54');
INSERT INTO test1 VALUES ('4', '李四', '數學', '134');
INSERT INTO test1 VALUES ('5', '李四', '語文', '144');
INSERT INTO test1 VALUES ('6', '李四', '英語', '154');
INSERT INTO test1 VALUES ('7', '王五', '數學', '234');
INSERT INTO test1 VALUES ('8', '王五', '語文', '244');
INSERT INTO test1 VALUES ('9', '王五', '英語', '254');

查出如下結果

法1

SELECT
    A.username,A.score as '數學',B.score as '語文',C.score as '英語'
FROM 
(select username,course,score from test1 where course = '數學') A,
(select username,course,score from test1 where course = '語文') B,
(select username,course,score from test1 where course = '英語') C
WHERE
    A.username = B.username
and B.username = C.username

法2【推薦】

select
    username,sum(case course when '數學' then score else 0 end ) as '數學',
sum(case course when '語文' then score else 0 end ) as '語文',
sum(case course when '英語' then score else 0 end ) as '英語'
FROM
    test1
group by username