|CAST(expr AS type)|
CAST allows you to convert from one type to another.
Often a CAST is implicit - for example if you concatenate a string with a number the number will be automatically changed to a string. However sometimes you need to make the CAST explicit.
CAST(expr TO type)
In this example we get the population in millions by casting the floating point value to DECIMAL(8,1) - this ensures one decimal place of accuracy. You can also CAST a date to a string to extract components using SUBSTRING or make up another date.
SELECT CAST(population/1000000 AS DECIMAL(8,1)) AS a ,population/1000000 AS b FROM bbc