January 2019
Beginner
556 pages
14h 19m
English
Content preview from Learning PostgreSQL 11 - Third Edition
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Grouping and aggregation
In all of the previous examples, the number of records returned by the SELECT query was the same as the number of rows from the input table (or tables) after filtering. In other words, every row from any of the source tables (or joined tables) becomes exactly one row in the query result. Rows are processed one by one.
SQL provides a way to get aggregated results of processing several records at a time and then get the results in a single row. The easiest example would be counting the total number of records in a table. The input is all the records of a table. The output is a single record. Grouping and aggregation is used for this.
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