help pls... how to convert iginited datatable server side query?

Hey i'm writing a script at the moment and i need to write a query which will work like this

SELECT * FROM `table`
IF `table`.`field_1` = 'somthing'
LEFT JOIN `another_table`
ON `table`.`field_2` = `another_table`.`field_1`
ELSE IF `table`.`field_1` = 'somthing_else'
LEFT JOIN `another_table_2`
ON `table`.`field_2` = `another_table_2`.`field_1`
END IF
WHERE `table`.`field_3` = 'keyword'

I hope that makes sense. I've never used IF statements in mysql before and i can't find much documentation that helps.

Thanks