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How to Use string_agg()

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MS-SQL Server provides a function called string_agg, (in fact there are two functions with the same meaning, one is an aggregate function and the other is a window function). Here we cover the aggregated function. The idea is to concat several strings, from different rows separated by a specified separator.

create table teams (team_name varchar(20), team_member varchar(20));
insert into teams values ('Barcelona','Messi'), ('Barcelona','Piquet'), ('Barcelona','Xavi');
insert into teams values ('Real Madrid','Ronaldo'), ('Real Madrid','Benzema'), ('Real Madrid','Ramos');

This is our base table:

select * from teams;

`team_name`	`team_member`
`Real Madrid`	`Ronaldo`
`Real Madrid`	`Benzema`
`Real Madrid`	`Ramos`
`Barcelona`	`Messi`
`Barcelona`	`Piquet`
`Barcelona`	`Xavi`

This function concatenates the values of a given column using a separator, because LISTAGG is an aggregate function, the group by clause plays an important role to define what rows are grouped.

select team_name, string_agg(team_member,', ') AS members
from   teams 
group by team_name;

`team_name`	`members`
`Real Madrid`	`Ronaldo, Benzema, Ramos`
`Barcelona`	`Messi, Piquet, Xavi`

The group by team_name clause creates groups of rows with the same value in team_name, then all the members of the same team are concatenated in the same string.