groupconcatenation

Group concatenation is a data aggregation operation that combines multiple string values from rows into a single string for each group defined by a grouping key. It is commonly used to collapse many rows into a list per group, such as listing all employees in a department or all tags associated with a post. The exact function name and syntax vary by database system, but the core idea is the same: aggregate strings with a delimiter to produce one concatenated result per group.

Common implementations by database system include: MySQL uses GROUP_CONCAT, PostgreSQL uses STRING_AGG, Oracle uses LISTAGG, and SQL Server also offers STRING_AGG. These functions typically accept a string expression to concatenate, a delimiter to insert between values, and an optional ORDER BY clause to control the sort order within the group. Many implementations also support DISTINCT to remove duplicates.

Examples (conceptual):

- MySQL: SELECT department_id, GROUP_CONCAT(employee_name ORDER BY employee_name SEPARATOR ', ') AS employees FROM employees GROUP BY department_id;

- PostgreSQL: SELECT department_id, STRING_AGG(employee_name, ', ' ORDER BY employee_name) AS employees FROM employees GROUP BY department_id;

Limitations and considerations include maximum result length in some systems (for example, MySQL has a configurable group_concat_max_len), potential performance costs on large datasets, and dialect-specific variations in syntax. NULLs are typically ignored by the concatenation, and using DISTINCT can remove duplicates within the aggregated list.

Uses of group concatenation include creating denormalized lists for reports, generating comma-separated item lists, and enabling compact display of related values within a single field per group. It is a widely used, though non-standard, feature of SQL-based databases to summarize one-to-many relationships.