SQL Subqueries: Single Row Subqueries Explained

What Are Single Row Subqueries?

A single row subquery returns exactly one row with one column. This makes it suitable for use with comparison operators in SQL statements.

If a subquery returns more than one row, SQL will raise an error when used in a context expecting a single value.

• Return one row and one column
• Used with operators like =, <, >, <=, >=, <>
• Commonly found in WHERE or HAVING clauses

Syntax of Single Row Subqueries

The general syntax involves placing a SELECT statement inside parentheses, which returns a single value.

This subquery is then compared to a column or value in the outer query.

• SELECT column_name FROM table WHERE column operator (subquery);
• The subquery must return only one value to avoid errors.

Examples of Single Row Subqueries

Let's look at practical examples to understand how single row subqueries work.

Common Errors with Single Row Subqueries

A common mistake is using a subquery that returns multiple rows where only one is expected.

This causes SQL errors like 'subquery returns more than one row'.

• Ensure subquery returns only one row and one column.
• Use aggregate functions like MAX(), MIN(), AVG() to guarantee single value.
• Use LIMIT 1 if supported and appropriate.