Consider the following SQL statement:SELECT employee_id FROM employeesUNIONSELECT employee_id FROM former_employees;What does this statement do
Question
Consider the following SQL statement:SELECT employee_id FROM employeesUNIONSELECT employee_id FROM former_employees;What does this statement do
Solution
This SQL statement is combining the results of two select queries using the UNION operator.
Here's a step-by-step breakdown:
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SELECT employee_id FROM employees: This part of the statement selects theemployee_idfrom theemployeestable. It will return a list of all current employee IDs. -
SELECT employee_id FROM former_employees: This part of the statement selects theemployee_idfrom theformer_employeestable. It will return a list of all former employee IDs. -
UNION: This is a set operator that combines the result sets of two or more SELECT statements. It removes duplicate rows from the result sets.
So, the entire statement SELECT employee_id FROM employees UNION SELECT employee_id FROM former_employees; will return a list of employee IDs from both the employees and former_employees tables, removing any duplicates. This means if an employee ID is present in both tables, it will appear only once in the final result set.
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