Sql case is a conditional option for query.It use like if-then-else in sql server.In this you can use multiple when and then like if else condition.For example i have written syntax in below.In example there are two when codition are given, but you can use multiple when in the query. sql case can be used in select,update,search,having and order by.

syntax of sql server case:

sql case in select statement

SELECT   ProductNumber, Category =
      CASE ProductLine
         WHEN 'R' THEN 'Road'
         WHEN 'M' THEN 'Mountain'
         WHEN 'T' THEN 'Touring'
         WHEN 'S' THEN 'Other sale items'
         ELSE 'Not for sale'
      END,
   Name
FROM Production.Product
ORDER BY ProductNumber;

sql case statement for search

SELECT   ProductNumber, Name, 'Price Range' =
      CASE
         WHEN ListPrice =  0 THEN 'Mfg item - not for resale'
         WHEN ListPrice < 50 THEN 'Under $50'
         WHEN ListPrice >= 50 and ListPrice < 250 THEN 'Under $250'
         WHEN ListPrice >= 250 and ListPrice < 1000 THEN 'Under $1000'
         ELSE 'Over $1000'
      END
FROM Production.Product
ORDER BY ProductNumber ;

sql case statement in order by

SELECT EmployeeID, SalariedFlag
FROM HumanResources.Employee
ORDER BY CASE SalariedFlag WHEN 1 THEN EmployeeID END DESC
        ,CASE WHEN SalariedFlag = 0 THEN EmployeeID END;
GO

sql case in update statement

UPDATE HumanResources.Employee
SET VacationHours =
    ( CASE
         WHEN ((VacationHours - 10.00) < 0) THEN VacationHours + 40
         ELSE (VacationHours + 20.00)
       END
    )
OUTPUT Deleted.EmployeeID, Deleted.VacationHours AS BeforeValue,
       Inserted.VacationHours AS AfterValue
WHERE SalariedFlag = 0;

sql case in a HAVING clause

SELECT Title, MAX(ph1.Rate)AS MaximumRate
FROM HumanResources.Employee AS e
JOIN HumanResources.EmployeePayHistory AS ph1 ON e.EmployeeID = ph1.EmployeeID
GROUP BY Title
HAVING (MAX(CASE WHEN Gender = 'M'
        THEN ph1.Rate
        ELSE NULL END) > 40.00
     OR MAX(CASE WHEN Gender  = 'F'
        THEN ph1.Rate
        ELSE NULL END) > 42.00)
ORDER BY MaximumRate DESC;