How Can I Efficiently Aggregate a Column in a Multi-Column Query?

Aggregate a Column in Query with Numerous Other Columns

When querying with several columns in the result set, aggregating a single column can be a challenge. While you could use the method mentioned in the referenced answer, it may lead to verbose queries.

Simplifying the Query (PostgreSQL 9.1 and Later)

PostgreSQL 9.1 and later provides a simpler approach. By grouping by a table's primary key, you can aggregate the specified column without having to include all columns in the GROUP BY clause.

For example, if foo1 is a primary key in your sample query:

SELECT foo1, foo2, foo3, foo4, string_agg(aggregated_field, ', ')
FROM tbl1
GROUP BY 1
ORDER BY foo5, foo6

Multiple Tables and Relationships

When your query involves multiple tables with various relationships, it's often more efficient to aggregate first and then join:

SELECT t1.foo1, t1.foo2, ..., t2.bar1, t2.bar2, ..., a.aggregated_col
FROM tbl1 t1
LEFT JOIN tbl2 t2 ON ...
...
LEFT JOIN (
   SELECT some_id, string_agg(agg_col, ', ') AS aggregated_col
   FROM agg_tbl a ON ...
   GROUP BY some_id
   ) a ON a.some_id = ?.some_id
ORDER BY ...

By separating the aggregation and join operations, your query becomes more efficient and easier to read.

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