How Does SQL Handle Row Value Comparisons in WHERE Clauses?

A detailed explanation of row values ​​in SQL

In the WHERE clause of SQL, row values ​​are often compared, such as (col1, col2) < (val1, val2). This article will delve into the syntax and functionality of this row value comparison, as well as its application in database queries.

Core Concepts

• Row Values: A row of data in a database table, containing the values ​​of multiple columns.
• Row Value Comparison: Compare two rows of data to determine their size relationship.
• Row Constructor Comparison: Use row constructor (col1, col2) to perform row value comparison.
• Row-wise Comparison: Compare row values ​​one by one in column order.

This syntax is often used in "keyset pagination" to retrieve data records before or after a specific row.

Syntax and functionality

The expression (col1, col2) < (val1, val2) means: if col1 < val1, the expression is true; if col1 = val1 and col2 < val2, the expression is also true; otherwise, it is false.

This is different from the more verbose equivalent expression:

(col1 < val1) OR (col1 = val1 AND col2 < val2)

PostgreSQL can leverage multi-column indexes on (col1, col2) or (col1 DESC, col2 DESC) to optimize such queries. Note that row value comparison is not the same as:

col1 < val1 AND col2 < val2

The difference is obvious when considering row values ​​like (1,1).

More resources

For more information, please refer to Markus Winand's presentation, "The Correct Way to Paginate in PostgreSQL":

The correct way to paginate in PostgreSQL

Row value comparison is discussed on page 20 and the supporting matrix is ​​provided on page 45.

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