case when in sql server's stored procedure

https://docs.microsoft.com/en-us/sql/t-sql/language-elements/case-transact-sql

Evaluates a list of conditions and returns one of multiple possible result expressions.

The CASE expression has two formats:

• The simple CASE expression compares an expression to a set of simple expressions to determine the result.

• The searched CASE expression evaluates a set of Boolean expressions to determine the result.

  Both formats support an optional ELSE argument.

  CASE can be used in any statement or clause that allows a valid expression. For example, you can use CASE in statements such as SELECT, UPDATE, DELETE and SET, and in clauses such as select_list, IN, WHERE, ORDER BY, and HAVING.

-- Syntax for SQL Server and Azure SQL Database  

Simple CASE expression:
CASE input_expression
     WHEN when_expression THEN result_expression [ ...n ]
     [ ELSE else_result_expression ]
END
Searched CASE expression:
CASE
     WHEN Boolean_expression THEN result_expression [ ...n ]
     [ ELSE else_result_expression ]
END  

case必须要和end配套，一个开头一个结尾

case 表达式

中间的内容

end

中间的内容

when  表达式的结果 then 处理逻辑

else（else是一个可选项，可有可无）