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SQL for Data Analytics - Third Edition
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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.
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...