Cleaning Data

Often, the raw data presented in a query output may not be in the desired form. You may want to remove values, substitute values, or map values to other values. To accomplish these tasks, SQL provides a wide variety of statements and functions. Functions are keywords that take in inputs (such as a column or a scalar value) and process those inputs into some sort of output. You will learn about some useful functions for data transformation and cleaning in the following sections.

The CASE WHEN Function

CASE WHEN is a function that allows a query to map various values in a column to other values. The general format of a CASE WHEN statement is as follows:

CASE 
  WHEN condition1 THEN value1
  WHEN condition2 THEN value2
  …
  WHEN conditionX THEN valueX
  ELSE else_value 
END;

Here, condition1 and condition2, through conditionX, are Boolean conditions; value1 and value2, through valueX, are values to map...