题目

找出每一个球员赢得大满贯比赛的次数。结果不包含没有赢得比赛的球员的ID 。

结果集 无顺序要求 。

准备数据

Create table If Not Exists Players (player_id int, player_name varchar(20));
Create table If Not Exists Championships (year int, Wimbledon int, Fr_open int, US_open int, Au_open int);
Truncate table Players;
insert into Players (player_id, player_name) values ('1', 'Nadal');
insert into Players (player_id, player_name) values ('2', 'Federer');
insert into Players (player_id, player_name) values ('3', 'Novak');
Truncate table Championships;
insert into Championships (year, Wimbledon, Fr_open, US_open, Au_open) values ('2018', '1', '1', '1', '1');
insert into Championships (year, Wimbledon, Fr_open, US_open, Au_open) values ('2019', '1', '1', '2', '2');
insert into Championships (year, Wimbledon, Fr_open, US_open, Au_open) values ('2020', '2', '1', '2', '2');

Championships表

Players表

 分析数据

类型是行转列 一般要使用union（all）

 第一步:将几行转成一列,使用union all

select Wimbledon from Championships
union all
select Fr_open from Championships
union all
select US_open from Championships
union all
select Au_open from Championships;

第二步:将两张表进行关联

select player_id,player_name,count(*) as grand_slams_count
from players join
    (select Wimbledon from Championships
      union all
      select Fr_open from Championships
      union all
      select US_open from Championships
      union all
      select Au_open from Championships) t1
    on t1.Wimbledon = player_id
group by player_id, player_name;

总结

• 若是行转列,使用union all
• 若是列转行, 利用if函数将数据拉宽