How Does MySQL's Null-Safe Equality Operator `` Work?

MySQL's Null-Safe Equality Operator: Understanding the <=> Operator

When working with database queries, understanding the operators used is essential. In MySQL, the operator <=> is often encountered, raising questions about its meaning and functionality.

Query Context

In the provided query fragment:

WHERE p.name <=> NULL

the <=> operator is used to compare the value of p.name against NULL. This comparison raises a fundamental question: what does the operator actually do?

Null-Safe Equality

Unlike the regular equality operator (=), the <=> operator is known as the null-safe equality operator. It performs an equality comparison, but it treats the value of NULL differently.

Behavior with Non-NULL Values

When comparing non-NULL values, <=> behaves like the = operator. Two values are compared, and the result is either 0 (not equal) or 1 (equal). For example:

'a' <=> 'b' = 0 (not equal)
'a' <=> 'a' = 1 (equal)

Behavior with NULL Values

However, where <=> truly stands out is its handling of NULL values. With <=>, values of NULL don't have a special meaning. The comparison always yields 0 (not equal) or 1 (equal) without the possibility of a NULL outcome. For instance:

'a' <=> NULL = 0 (not equal)
NULL <=> NULL = 1 (equal)

Usefulness and Applications

The null-safe equality operator is particularly useful when both operands may contain NULL. It provides a consistent comparison result between two columns, regardless of whether they contain NULL. Additionally, it can be used with prepared statements to conveniently handle either scalar values or NULL placeholders.

Related Operators and Alternatives

Besides <=>, MySQL also provides two other operators specifically designed for comparisons against NULL: IS NULL and IS NOT NULL. These operators are part of the ANSI standard and are supported on multiple databases.

The SQL:2003 standard introduced the predicate IS [NOT] DISTINCT FROM, which functions identically to MySQL's <=> operator. However, its support is not universal.

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