SQL questions

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1- How to find duplicates in a table
2- How to delete duplicates from a table
3- Difference between union and union all
4- Difference between rank,row_number and dense_rank
5- Find records in a table which are not present in another table
6- Find second highest salary employees in each department
7- Find employees with salary more than their manager's salary
8- Difference between inner and left join
9- update a table and swap gender values.

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