Summary: The Postgres concat() function is a variadic string function that joins two or more strings or values into a single string, automatically converting numeric arguments and silently treating NULL inputs as empty strings. Use it instead of the || operator when NULL values should be ignored rather than propagate through the result. The function accepts either individual arguments or a variadic array form (concat(variadic arr)), covering use cases such as combining column values, generating composite keys, and building dynamic labels.

Concatenate strings in Postgres with the concat() function

The concat() function in Postgres is used to concatenate two or more strings into a single string. It is a variadic function, meaning it can accept any number of arguments.

It is useful for combining data from multiple columns, generating custom identifiers or labels, or constructing dynamic SQL statements.

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The concat() function has two forms:

concat(str "any" [, str "any" [, ...] ]) → text

• str: The strings/values to concatenate. Numeric values are automatically converted to strings, while NULL values are treated as empty strings.

concat(variadic str "any"[]) → text

• variadic str: An array of strings/values to concatenate. This form is useful when you have an array of strings to concatenate.

Consider a table customers with first_name and last_name columns. We can use concat() to combine these into a full name.

WITH customers AS (
  SELECT 'John' AS first_name, 'Doe' AS last_name
  UNION ALL
  SELECT 'Jane' AS first_name, 'Smith' AS last_name
)
SELECT concat(first_name, ' ', last_name) AS full_name
FROM customers;

This query concatenates the first_name, a space character, and the last_name to generate the full_name.

  full_name
-------------
 John Doe
 Jane Smith
(2 rows)

We can concatenate more than two strings by providing additional arguments.

WITH products AS (
  SELECT 'Laptop' AS name, 'A' AS variant, 100 AS price
  UNION ALL
  SELECT 'Kindle' AS name, NULL AS variant, 200 AS price
  UNION ALL
  SELECT 'Table' AS name, 'C' AS variant, 300 AS price
)
SELECT concat(name, CASE WHEN variant IS NOT NULL THEN ' - Variant ' ELSE '' END, variant, ' ($', price, ')') AS product_info
FROM products;

This query generates a descriptive product_info string by concatenating the name, variant, and price columns along with some constant text. We used a CASE statement to conditionally include the variant in the output.

       product_info
---------------------------
 Laptop - Variant A ($100)
 Kindle ($200)
 Table - Variant C ($300)
(3 rows)

You can use the variadic form of concat() to concatenate an array of strings.

WITH data AS (
  SELECT ARRAY['apple', 'banana', 'cherry'] AS fruits
)
SELECT concat(variadic fruits) AS fruit_string
FROM data;

This query concatenates the elements of the fruits array into a single string.

  fruit_string
----------------
 applebananacherry
(1 row)

concat() can be used to generate custom identifiers as keys, which you can use for further processing or analysis.

WITH page_interactions AS (
  SELECT 1 AS user_id, '/home' AS page, '2023-06-01 10:00:00' AS ts
  UNION ALL
  SELECT 1 AS user_id, '/products' AS page, '2023-06-01 10:30:00' AS ts
  UNION ALL
  SELECT 2 AS user_id, '/home' AS page, '2023-06-01 11:00:00' AS ts
  UNION ALL
  SELECT 1 AS user_id, '/home' AS page, '2023-06-01 12:00:00' AS ts
)
SELECT unique_visit, count(*) AS num_interactions
FROM (
    SELECT ts, concat(user_id, ':', page) AS unique_visit
    FROM page_interactions
)
GROUP BY unique_visit;

This query generates a unique identifier for each page visit by concatenating the user_id and page columns. We then count the number of interactions for each unique visit.

 unique_visit | num_interactions
--------------+------------------
 1:/home      |                2
 2:/home      |                1
 1:/products  |                1
(3 rows)

Any null arguments to concat() are treated as empty strings in the output. This is in contrast to the behavior of the || operator, which treats NULL values as NULL.

SELECT
    concat('Hello', NULL, 'World') AS join_concat,
    'Hello' || NULL || 'World' AS join_operator;

Pick the right function based on how you want to handle NULL values.

 join_concat | join_operator
-------------+---------------
 HelloWorld  |
(1 row)

• concat_ws: Concatenates strings with a separator string between each element.
• string_agg: An aggregation function that combines strings from a column into a single string with a separator.
• || operator: Can also be used to concatenate strings. It treats NULL values differently than concat().

• PostgreSQL documentation: String functions

• Aggregate functions
• Array functions
• Date / Time functions
• JSON functions
• Window functions
• Math functions

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