In SQL, the less than operator (<) compares two expressions and returns TRUE if the left operand has a value lower than the right operand; otherwise, the result is FALSE.
SQL
SQL Not Equal To (<>) Operator for Beginners
In SQL, the not equal to operator (<>) compares the non-equality of two expressions. That is, it tests whether one expression is not equal to another expression.
If either or both operands are NULL, NULL is returned.
SQL also has another not equal to operator (!=), which does the same thing. Which one you use may depend on your DBMS, which one you’re the most comfortable using, and perhaps also whether your organisation has any coding conventions that dictate which one should be used.
SQL Not Equal To (!=) Operator for Beginners
In SQL, the not equal to operator (!=) compares the non-equality of two expressions. That is, it tests whether one expression is not equal to another expression.
If either or both operands are NULL, NULL is returned.
SQL also has another not equal to operator (<>), which does the same thing. Which one you use may depend on your DBMS, which one you’re the most comfortable using, and perhaps also whether your organisation has any coding conventions that dictate which one should be used.
SQL EXISTS Operator for Beginners
In SQL, the EXISTS operator specifies a subquery to test for the existence of rows. It returns TRUE when the subquery returns one or more rows.
A subquery is a query that is nested inside another query (or even another subquery)
This article contains some basic examples of the EXISTS operator.