Can a MySQL query turn rows into columns?

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醉话见心
醉话见心 2020-12-01 10:25

I have a number of tables I am trying to combine with joins but as such, the results are returned in a number of rows whereas I would like to have them generated as new colu

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  •  独厮守ぢ
    2020-12-01 10:46

    Sometimes the first step to solving your problem is knowing what it's called. After that, it's simply a matter of googling. What you are trying to create is called a pivot table or crosstab report. Here is a link explaining how to create pivot tables in MySQL. And here is a more in depth tutorial.

    UPDATE:

    Now that you've updated the question, I have a clearer idea of what you are trying to accomplish. I'll give you an alternative solution which is similar but not exactly what you want based on MySQL's GROUP_CONCAT function.

    select t1.FirstName, t1.LastName, group_concat(concat(t2.FirstName, ' ', t2.LastName))
    from member_information as t1
    left outer join member_dependent_information as t2 on t2.MemberID=t1.MemberID
    group by t1.MemberID;
    

    I've verified this query as follows. First the setup:

    create table member_information (
        MemberID int unsigned auto_increment primary key,
        FirstName varchar(32) not null,
        LastName varchar(32) not null
    ) engine=innodb;
    
    create table member_dependent_information (
        MemberID int unsigned not null,
        FirstName varchar(32) not null,
        LastName varchar(32) not null,
        Type int unsigned not null,
        foreign key (MemberID) references member_information(MemberID)
    ) engine=innodb;
    
    insert into member_information (MemberID, FirstName, LastName) values
    (1, 'John', 'Harris'),
    (2, 'Sarah', 'Thompson'),
    (3, 'Zack', 'Lewis');
    
    insert into member_dependent_information (MemberID, FirstName, LastName, `Type`) values
    (1, 'Amy', 'Harris', 1),
    (2, 'Bryan', 'Thompson', 1),
    (2, 'Dewey', 'Thompson', 2),
    (2, 'Tom', 'Thompson', 2),
    (3, 'Harry', 'Lewis', 2),
    (3, 'Minka', 'Lewis', 1);
    

    And now the query and results:

    mysql> select t1.FirstName, t1.LastName, group_concat(concat(t2.FirstName, ' ', t2.LastName))from member_information as t1
        -> left outer join member_dependent_information as t2 on t2.MemberID=t1.MemberID
        -> group by t1.MemberID;
    +-----------+----------+------------------------------------------------------+
    | FirstName | LastName | group_concat(concat(t2.FirstName, ' ', t2.LastName)) |
    +-----------+----------+------------------------------------------------------+
    | John      | Harris   | Amy Harris                                           | 
    | Sarah     | Thompson | Bryan Thompson,Dewey Thompson,Tom Thompson           | 
    | Zack      | Lewis    | Harry Lewis,Minka Lewis                              | 
    +-----------+----------+------------------------------------------------------+
    3 rows in set (0.00 sec)
    

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