What makes a Scalar Subquery particularly restrictive?

A Scalar Subquery is defined by its ability to return exactly one value. This distinguishes it from other types of subqueries, which may return multiple rows or columns. Because it is designed to yield a single value, a Scalar Subquery is particularly useful in contexts where precise, singular results are necessary, such as in condition checks for WHERE clauses or as part of an expression in a SELECT statement.

The restriction to a single value ensures that the subquery can seamlessly integrate into larger queries without causing ambiguity or errors associated with multiple returned values. This characteristic makes Scalar Subqueries a strong choice for scenarios requiring a compact and clear return type, like defining a threshold or comparing a single value against the results of another query.

In contrast, returning multiple rows would lead to an error in situations expecting a single value, and the focus on a specific data type adds clarity but isn't inherently part of what defines a Scalar Subquery's restrictive nature. Moreover, while Scalar Subqueries can work with aggregated data, they are not limited to aggregation, thus highlighting their versatility alongside their specificity.