Question

我试图在mysql查询中使用case语句将行转换为列。我在这里看到过这样的其他问题，但由于某些原因我不能让它工作。这是示例原始数据。

last_name  question_number  result
Jones        1               correct
Jones        2               correct
Jones        3               incorrect
Jones        4               correct
Smith        1               incorrect
Smith        2               correct
Smith        3               correct
Smith        4               incorrect

Here is what I want to end up with

last_name    Q1         Q2         Q3         Q4
Jones        correct    correct    incorrect  correct
Smith        incorrect  correct    correct    incorrect

这是设置，我正在尝试...

drop table if exists test_case;
create table test_case (
    last_name varchar(30),
    question_number int,
    result varchar(30)
);

insert into test_case 
values ('Jones',1,'correct'), ('Jones',2,'correct'), ('Jones',3,'incorrect'), ('Jones',4,'correct'),
('Smith',1,'incorrect'), ('Smith',2,'correct'), ('Smith',3,'correct'), ('Smith',4,'incorrect');

select * from test_case;

select 
    last_name,
    case
        when question_number = 1 then result
        else null
    end as Q1,
    case
        when question_number = 2 then result
        else ''
    end as Q2,
    case
        when question_number = 3 then result
        else ''
    end as Q3,
    case
        when question_number = 4 then result
        else ''
    end as Q4
from
    test_case
group by last_name;

然而我得到的却是......

 last_name    Q1         Q2         Q3         Q4
Jones        correct    
Smith        incorrect

Answer 1

您的查询中需要聚合函数：

select last_name,
       max(case when question_number = 1 then result end) as Q1,
       max(case when question_number = 2 then result end) as Q2,
       max(case when question_number = 3 then result end) as Q3,
       max(case when question_number = 4 then result end) as Q4
from test_case
group by last_name;

在大多数数据库中，您的查询版本无法编译。如果question_number子句中没有group by，则会收到错误。 MySQL允许语法，但只返回单行的任意值。 max()确保处理所有行。

使用case将列值转换为mysql中的行值

1 个答案: