How to Concatenate Field Values in PostgreSQL for Multiple Records with a Single ID?

Concatenating field values ​​in PostgreSQL for multiple records and a single ID

Many database applications require combining multiple values ​​related to a single identifier into a single connection string. This operation is particularly useful when querying a table with multiple records per unique ID.

GROUP_CONCAT function in MySQL

In MySQL, the GROUP_CONCAT function provides a direct way to achieve this goal. However, the GROUP_CONCAT function is not available directly in PostgreSQL.

PostgreSQL equivalent of GROUP_CONCAT

In PostgreSQL, the string_agg function is the equivalent of MySQL's GROUP_CONCAT. This function allows you to concatenate strings while specifying a delimiter (such as a comma) to separate the concatenated values.

Example: PostgreSQL query using string_agg

Consider the example table provided in the original question:

id	some_column
TM67	4
TM67	9
TM67	72
TM99	2
TM99	3

The following PostgreSQL query can be used to concatenate the values ​​of some_column columns for each unique id:

<code class="language-sql">SELECT
   id,
   string_agg(some_column::text, ',') AS values
FROM
   the_table
GROUP BY
   id;</code>

Result:

id	values
TM67	4,9,72
TM99	2,3

This query retrieves one row for each unique id and concatenates the values ​​of some_column columns into a single string with commas as delimiters.

Note: The string_agg function was introduced in PostgreSQL version 9.0. For earlier versions of PostgreSQL, you can use the generate_series or group_concat_agg extensions as alternatives to achieve the same functionality. The type conversion of some_column::text ensures that even if some_column is not a text type, it will be concatenated correctly.

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