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There are some rules to be followed when using a correlated sub query:

• Must be enclosed in parentheses
• Can be used with the IN or NOT IN clauses
• Can be used with the EXISTS or NOT EXISTS clauses
• Supports quantifiers such as ALL, ANY, and SOME
• Can use LIKE or NOT LIKE, used with a quantifier
• Can specify more than one column to match
• Gives a DISTINCT list of values
• ORDER BY can't be used
• Maximum of 64 tables/views can be specified in an SQL statement