Usage of case when statement in sql

The CASE WHEN statement is used in SQL to evaluate an expression and return a different value based on specified conditions. Its syntax consists of WHEN clause (conditional expression and return expression), ELSE clause (default expression) and END keyword. It can be used in a variety of scenarios, including assigning values, modifying results, and converting data formats.

Usage of CASE WHEN statement in SQL

CASE WHEN statement is a control flow in SQL query Statement that evaluates different expressions based on specified conditions. The syntax is as follows:

<code class="sql">CASE
  WHEN <condition 1> THEN <expression 1>
  WHEN <condition 2> THEN <expression 2>
  ...
  ELSE <default_expression>
END</code>

Usage:

• Condition: Specifies the Boolean expression used to evaluate the expression.
• Expression: The value that is evaluated and returned if the condition is true.
• Default_expression: The value that is evaluated and returned when no condition is true.

Steps:

1. Determine the condition to evaluate: Determine what condition you want to return a different value based on.
2. Writing the WHEN clause: For each condition, use the WHEN clause to specify the condition and the expression to be returned.
3. Add ELSE clause: Specify the default return value when all conditions are not met (optional).
4. End the CASE statement: Use the END keyword to end the CASE statement.

Example:

To assign discounts to customers based on their age group, you can use the following SQL query:

<code class="sql">SELECT CASE
  WHEN age < 18 THEN 0.1
  WHEN age >= 18 AND age < 25 THEN 0.15
  WHEN age >= 25 AND age < 35 THEN 0.2
  ELSE 0.25
END AS discount
FROM customers;</code>

In this query Medium:

• condition groups customers based on their age.
• Expression specifies the discount rate for each age group.
• The default expression is 0.25, which represents the discount rate for all customers over 35 years old.

Use cases:

The CASE WHEN statement can be used in various scenarios, including:

• Assign different values ​​based on conditions ( such as discounts or categories).
• Modify query results based on conditions (such as filtering or sorting).
• Convert data formats (such as converting text to numbers).

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