MySQL GROUP_CONCAT and DISTINCT

Question

I have tb1 table like this :

name   email               link
john   myemail@gmail.com   google
john   myemail@gmail.com   facebook
john   myemail2@gmail.com  twitter
....
....
and more

When I call data with query looks like

SELECT name, email, group_concat(DISTINCT link SEPARATOR '/') as source
FROM tb1
group by email

And result Like this :

NAME    EMAIL               SOURCE
john    myemail2@gmail.com  twitter
john    myemail@gmail.com   facebook/google
....
....
and more

sqlfiddle

I want make result looks like :

NAME    EMAIL               SOURCE 1       SOURCE 2
john    myemail2@gmail.com  twitter
john    myemail@gmail.com   facebook       google

It's possible to make result only with query?

Note : I want to make dynamic column source 1, 2, 3 ...., n

Answer 1

My answer is, don't do it!

What you want to do is called pivoting. In MySQL this is usually done with one hell of a

....
aggregate_function(case when ...),
aggregate_function(case when ...),
aggregate_function(case when ...),
...

block. But since you don't know all your sources, you'd have to write a procedure to dynamically build the statement for you and execute it. And even then you'd end up with a table structure like

NAME    EMAIL               twitter     facebook    google
----------------------------------------------------------
john    myemail2@gmail.com  yes         no          no
john    myemail@gmail.com   no          yes         yes

So, how would the programmers of your application know how the columns are named?

To avoid this you'd have to do again one hell of a programming in your stored procedure to put the row values of each email into one column. Then how would you determine, which row gets into which column? What if facebook appears in one row in column source1 and in another row in column source2? And in your application you'd have to test or otherwise determine how much columns there are actually, or can each user/email only have 2 sources?

To sum things up, a result like this

name   email               link
john   myemail@gmail.com   google
john   myemail@gmail.com   facebook
john   myemail2@gmail.com  twitter

is perfectly fine and it's up to the application developers to deal with it. And it's much, much easier to do so for them than for the database guy in your team.

Answer 2

不，你不能用MySQL，你不能动态创建额外的列...（这是一个olap的东西，或者允许这样的功能称为数据透视表）

Answer 3

It can work for this case, but its not a dynamic query and can work only if there are maximum of 2 link present for same email.

SELECT `name`, email, 
substring_index(group_concat(DISTINCT link SEPARATOR '/'),'/',1) as source_1,
if(group_concat(DISTINCT link SEPARATOR '/') like '%/%',
substring_index(group_concat(DISTINCT link SEPARATOR '/'),'/',-1),'') as source_2
FROM test2 group by email

If you know the exact number of possibility of links present per email then the same query can be modify accordingly.