SQL CASE

From WikiOD

The CASE expression is used to implement if-then logic.

Syntax[edit | edit source]

  • CASE input_expression

 WHENN compare1 THEN result1

[WHEN compare2 THEN result2]...

[ELSE resultX]

END

  • CASE

 WHENN condition1 THEN result1

[WHEN condition2 THEN result2]...

[ELSE resultX]

END

Remarks[edit | edit source]

The simple CASE expression returns the first result whose compareX value is equal to the input_expression.

The searched CASE expression returns the first result whose conditionX is true.

Use CASE to COUNT the number of rows in a column match a condition.[edit | edit source]

Use Case

CASE can be used in conjunction with SUM to return a count of only those items matching a pre-defined condition. (This is similar to COUNTIF in Excel.)

The trick is to return binary results indicating matches, so the "1"s returned for matching entries can be summed for a count of the total number of matches.

Given this table ItemSales, let's say you want to learn the total number of items that have been categorized as "Expensive":

Id ItemId Price PriceRating
1 100 34.5 EXPENSIVE
2 145 2.3 CHEAP
3 100 34.5 EXPENSIVE
4 100 34.5 EXPENSIVE
5 145 10 AFFORDABLE

Query

SELECT 
    COUNT(Id) AS ItemsCount,
    SUM ( CASE 
            WHEN PriceRating = 'Expensive' THEN 1
            ELSE 0
          END
        ) AS ExpensiveItemsCount
FROM ItemSales

Results:

ItemsCount ExpensiveItemsCount
5 3

Alternative:

SELECT 
    COUNT(Id) as ItemsCount,
    SUM (
        CASE PriceRating 
            WHEN 'Expensive' THEN 1
            ELSE 0
        END
       ) AS ExpensiveItemsCount
FROM ItemSales

Searched CASE in SELECT (Matches a boolean expression)[edit | edit source]

The searched CASE returns results when a boolean expression is TRUE.

(This differs from the simple case, which can only check for equivalency with an input.)

SELECT Id, ItemId, Price,
  CASE WHEN Price < 10 THEN 'CHEAP'
       WHEN Price < 20 THEN 'AFFORDABLE'
       ELSE 'EXPENSIVE'
  END AS PriceRating
FROM ItemSales
Id ItemId Price PriceRating
1 100 34.5 EXPENSIVE
2 145 2.3 CHEAP
3 100 34.5 EXPENSIVE
4 100 34.5 EXPENSIVE
5 145 10 AFFORDABLE

CASE in a clause ORDER BY[edit | edit source]

We can use 1,2,3.. to determine the type of order:

SELECT * FROM DEPT
ORDER BY
CASE DEPARTMENT
      WHEN 'MARKETING' THEN  1
      WHEN 'SALES' THEN 2
      WHEN 'RESEARCH' THEN 3
      WHEN 'INNOVATION' THEN 4
      ELSE        5
      END,
      CITY
ID REGION CITY DEPARTMENT EMPLOYEES_NUMBER
12 New England Boston MARKETING 9
15 West San Francisco MARKETING 12
9 Midwest Chicago SALES 8
14 Mid-Atlantic New York SALES 12
5 West Los Angeles RESEARCH 11
10 Mid-Atlantic Philadelphia RESEARCH 13
4 Midwest Chicago INNOVATION 11
2 Midwest Detroit HUMAN RESOURCES 9

Shorthand CASE in SELECT[edit | edit source]

CASE's shorthand variant evaluates an expression (usually a column) against a series of values. This variant is a bit shorter, and saves repeating the evaluated expression over and over again. The ELSE clause can still be used, though:

SELECT Id, ItemId, Price,
  CASE Price WHEN 5  THEN 'CHEAP'
             WHEN 15 THEN 'AFFORDABLE'
             ELSE         'EXPENSIVE'
  END as PriceRating
FROM ItemSales

A word of caution. It's important to realize that when using the short variant the entire statement is evaluated at each WHEN. Therefore the following statement:

SELECT 
    CASE ABS(CHECKSUM(NEWID())) % 4
        WHEN 0 THEN 'Dr'
        WHEN 1 THEN 'Master'
        WHEN 2 THEN 'Mr'
        WHEN 3 THEN 'Mrs'
    END

may produce a NULL result. That is because at each WHEN NEWID() is being called again with a new result. Equivalent to:

SELECT 
    CASE 
        WHEN ABS(CHECKSUM(NEWID())) % 4 = 0 THEN 'Dr'
        WHEN ABS(CHECKSUM(NEWID())) % 4 = 1 THEN 'Master'
        WHEN ABS(CHECKSUM(NEWID())) % 4 = 2 THEN 'Mr'
        WHEN ABS(CHECKSUM(NEWID())) % 4 = 3 THEN 'Mrs'
    END

Therefore it can miss all the WHEN cases and result as NULL.

Using CASE in UPDATE[edit | edit source]

sample on price increases:

UPDATE ItemPrice
SET Price = Price *
  CASE ItemId
    WHEN 1 THEN 1.05
    WHEN 2 THEN 1.10
    WHEN 3 THEN 1.15
    ELSE 1.00
  END

CASE use for NULL values ​​ordered last[edit | edit source]

in this way '0' representing the known values ​​are ranked first, '1' representing the NULL values ​​are sorted by the last:

SELECT ID
      ,REGION
      ,CITY
      ,DEPARTMENT
      ,EMPLOYEES_NUMBER
  FROM DEPT
  ORDER BY 
  CASE WHEN REGION IS NULL THEN 1 
  ELSE 0
  END, 
  REGION
ID REGION CITY DEPARTMENT EMPLOYEES_NUMBER
10 Mid-Atlantic Philadelphia RESEARCH 13
14 Mid-Atlantic New York SALES 12
9 Midwest Chicago SALES 8
12 New England Boston MARKETING 9
5 West Los Angeles RESEARCH 11
15 NULL San Francisco MARKETING 12
4 NULL Chicago INNOVATION 11
2 NULL Detroit HUMAN RESOURCES 9

CASE in ORDER BY clause to sort records by lowest value of 2 columns[edit | edit source]

Imagine that you need sort records by lowest value of either one of two columns. Some databases could use a non-aggregated MIN() or LEAST() function for this (... ORDER BY MIN(Date1, Date2)), but in standard SQL, you have to use a CASE expression.

The CASE expression in the query below looks at the Date1 and Date2 columns, checks which column has the lower value, and sorts the records depending on this value.

Sample data[edit | edit source]

Id Date1 Date2
1 2017-01-01 2017-01-31
2 2017-01-31 2017-01-03
3 2017-01-31 2017-01-02
4 2017-01-06 2017-01-31
5 2017-01-31 2017-01-05
6 2017-01-04 2017-01-31

Query[edit | edit source]

SELECT Id, Date1, Date2
FROM YourTable
ORDER BY CASE 
           WHEN COALESCE(Date1, '1753-01-01') < COALESCE(Date2, '1753-01-01') THEN Date1 
           ELSE Date2 
         END

Results[edit | edit source]

Id Date1 Date2
1 2017-01-01 2017-01-31
3 2017-01-31 2017-01-02
2 2017-01-31 2017-01-03
6 2017-01-04 2017-01-31
5 2017-01-31 2017-01-05
4 2017-01-06 2017-01-31

Explanation[edit | edit source]

As you see row with Id = 1 is first, that because Date1 have lowest record from entire table 2017-01-01, row where Id = 3 is second that because Date2 equals to 2017-01-02 that is second lowest value from table and so on.

So we have sorted records from 2017-01-01 to 2017-01-06 ascending and no care on which one column Date1 or Date2 are those values.

Credit:Stack_Overflow_Documentation