<> is Standard SQL-92; != is its equivalent. Both evaluate for values, which NULL is not -- NULL is a placeholder to say there is the absence of a value. ... <看更多>

IS NULL , IS NOT NULL & IFNULL FUNCTIONSA field with a NULL value is a field with no value.If a field in a table is optional, it is possible ... ... <看更多>

In SQL a NULL value signifies an unknown value, so any comparison involving a NULL will produce NULL . The IS DISTINCT FROM and IS NOT DISTINCT FROM ... ... <看更多>

In short, when using a logical operator in combination with null , the result is always null . But in order to test for null , SQL comes with the IS (NOT) ... ... <看更多>