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How to implement ROW_NUMBER functionality in My-SQL

Sometimes while we're fetching result-set from underlying database we also want to get the row number for each and every row of this result-set. Generally it can easily be achieved by using RANKing Functions (e.g. ROW_NUMBER() OVER(ORDER BY <yourColumnName>).
But unfortunately my-sql doesn't support RANKing function.

Let's see below query how to get row number for the given result set.

CREATE TABLE testRowNumber
(
firstName VARCHAR(50) NOT NULL
)

INSERT INTO testRowNumber (firstName) VALUES
 ('Ram'), ('Shyam'), ('Mohan'), ('Sohan'), ('Sita'), ('Gita')

SELECT
   (@rowNum := @rowNum + 1) AS rowNumber
   , firstName
FROM testRowNumber AS e,
   (SELECT @rowNum := 0) a

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