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Definition of Subqueries A subquery occurs when one SQL statement contains multiple SELECT clauses. Consider this example: -- Example introducing subqueries: -- Find all employees with higher salaries than "JONES" -- Step 1: Get JONES's salary SELECT salary FROM staff WHERE name = 'JONES'; -- Returns 2975 -- Step 2: Find all employe ...

Database Constraints in DaMeng What Are Constraints? Constraints are rules enforced on table columns to insure data integrity. They guarantee both accuracy and validity of stored data. Constraint Types Primary Key Constraint A primary key uniquely identifies each row in a table. This field must contain unique, non-null values. Creating a table ...