How to Use string_agg() in PostgreSQL

string_agg() combines non-null values into one string, separated by the delimiter character that you specify in the second parameter. For example, in the Sakila database there's a city and a country table. If you want to show the available cities per country in one line, separated by commas:

select
  country,
  string_agg(city, ',') as cities
from country
join city using (country_id)
group by country
limit 4;
       country        |             cities             
----------------------+--------------------------------
 Thailand             | Songkhla,Nakhon Sawan,Pak Kret
 Faroe Islands        | Trshavn
 Bangladesh           | Jamalpur,Tangail,Dhaka
 United States        | Springfield, Springfield

Note that PostgreSQL does not assume a default delimiter. You need to specify it as the second parameter. Also note that like other aggregate functions, you must use the group by clause with string_agg().

Removing duplicates in our output string

Notice that in our results, there are multiple cities in the United States named "Springfield". If you want to omit any duplicates, simply add distinct in the first parameter:

select
  country,
  string_agg(distinct city, ',') as cities
from country
join city using (country_id)
group by country
limit 4;
       country        |             cities             
----------------------+--------------------------------
 Thailand             | Songkhla,Nakhon Sawan,Pak Kret
 Faroe Islands        | Trshavn
 Bangladesh           | Jamalpur,Tangail,Dhaka
 United States        | Springfield

Ordering the contents within the output string

Perhaps you also want the output of the string_agg() function to be ordered alphabetically. You can specify the order using order by after the second parameter:

select
  country,
  string_agg(distinct city, ',' order by city asc) as cities
from country
join city using (country_id)
group by country
limit 4;
       country        |             cities             
----------------------+--------------------------------
 Thailand             | Nakhon Sawan,Pak Kret,Songkhla
 Faroe Islands        | Trshavn
 Bangladesh           | Dhaka,Jamalpur,Tangail
 United States        | Springfield

This can be helpful for particular long outputs, but the order by clause is optional.

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