How Does MySQL's NULL-Safe Equals Operator () Work?

The Not Equal to NULL Operator in MySQL: <=>

When working with SQL queries, you might encounter the <=> operator, raising questions about its meaning and syntax. This guide will provide an in-depth understanding of this operator and its applications.

Understanding <=>

The <=> operator is known as the NULL-safe equal operator. It is analogous to the regular equal operator (=), but with a crucial distinction when dealing with NULL values.

Regular Equals Operator vs. NULL-Safe Equals Operator

The regular equal operator (=) returns 0 if the operands are unequal and 1 if they are equal. However, it treats NULL values as a special case, yielding NULL as a result in such comparisons.

The <=> operator, on the other hand, handles NULL values differently. It returns 0 if the operands are unequal and 1 if they are equal, regardless of whether either or both operands are NULL. In other words:

• 'a' <=> 'b' evaluates to 0
• 'a' <=> 'a' evaluates to 1
• 'a' <=> NULL evaluates to 0
• NULL <=> NULL evaluates to 1

Usefulness of <=>

The <=> operator is particularly useful in scenarios where both operands in a comparison may contain NULL values, and you need a consistent comparison result between two columns.

Another practical application of this operator is in prepared statements, where the placeholder can seamlessly handle both scalar values and NULL values without the need to modify the query.

Related Operators

MySQL also offers other operators for comparing values against NULL:

• IS NULL: Evaluates to 1 if the operand is NULL, and 0 otherwise.
• IS NOT NULL: Evaluates to 1 if the operand is not NULL, and 0 otherwise.

These operators can be considered specializations of MySQL's <=> operator. For example:

• 'a' IS NULL is equivalent to 'a' <=> NULL
• 'a' IS NOT NULL is equivalent to NOT('a' <=> NULL)

Support and Compatibility

The <=> operator is MySQL-specific and not supported on other databases.

The SQL:2003 standard introduced a predicate, IS [NOT] DISTINCT FROM, which behaves identically to MySQL's <=> operator.

Additionally, the following complex but universally supported expression can achieve the same results:

CASE WHEN (a = b) or (a IS NULL AND b IS NULL)
     THEN 1
     ELSE 0
END = 1

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