finding duplicate raws in a table

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Regarding duplicate rows in table, you could use DISTINCT keyword. For example Employee table has tons of records with details such as employee id, name, designation, department, etc., Now, if you want to display list of designations, use the following query.

SELCT DISTINCT EMP_DESIGNATION FROM TABLE_EMP

IN( & NOT IN) keywords are used to specify criteria in WHERE clause.

SELECT *
FROM TABLE_EMP
WHERE EMP_DESIGNATION IN ('Manager','Supervisor');

Above query will display all employees who're Manager or Supervisor. NOT IN works opposite way of IN like it'll display all employees who're not Manager or Supervisor.