Indexes in MySQL can improve performance of specific queries Like as below Examples :

Syntax for creating index on one field:

CREATE INDEX indx_magrno ON EmplyeTable (EmplyeID);

These two indexes is supposed to hugely improve performance of INNER JOINs like this one:

SELECT a.DeptNo, a.EmpNo, CONCAT(a.Name,a.Addrs) AS EmpName, CONCAT(b.Name,b.Addrs) AS ManagerDetails
FROM EmplyeTable AS a INNER JOIN EmplyeTable AS b ON a.EmplyeID = b.EmpNo
ORDER BY a.DeptNo;

Simple syntax for creating index on 3 address fields. it supposed to speed up address searching

CREATE INDEX indx_emp_address ON EmplyeTable (City,Street,HouseNum);

The same using ALTER TABLE syntax:

ALTER TABLE EmplyeTable ADD INDEX indx_emp_address (City,Street,HouseNum);

This index have to accelerate the execution of the following query:

SELECT EmpNo, Name, Addrs, City, Street, HouseNum
FROM EmplyeTable
WHERE City = 'Test' AND Street = 'Find Nerd';

or of the following JOIN:

SELECT a.EmpNo, a.Name, a.Addrs, a.City, b.x, b.y
FROM EmplyeTable AS a INNER JOIN Locations AS b
ON a.City = b.City
ORDER BY a.City;

If you do not need this index any more, drop it:

ALTER TABLE EmplyeTable DROP INDEX indx_emp_address;