MySQL: Condition based sorting

AscDescIntrioduction:

Sometimes we need to sort a result set based on some conditions. Let’s take few real time scenarios and than try to resolve these with the help of CASE WHEN. Scenario-1: Sometimes we need an element to be placed at bottom of the list. For example, a Dropdownlist containing educational qualifications may contain  an item “Other”. While showing all the educational qualifications from database we may need this particular element (i.e, “Other”) to be moved to the bottom of the list. If we try to sort this list using usual ORDER BY keyword then we may not get the desired sequence. To get the desired result we need to use CASE WHEN keyword along with the ORDER BY. Let’s illustrate this with an example- Simple ORDER BY :


SELECT * FROM
(
 SELECT 'I.Sc.' AS Qualification
 UNION
 SELECT 'B.Sc.' AS Qualification
 UNION
 SELECT 'Other' AS Qualification
 UNION
 SELECT 'P.G.' AS Qualification
 UNION
 SELECT 'MCA' AS Qualification
 UNION
 SELECT 'DCA' AS Qualification
 UNION
 SELECT 'P.H.D' AS Qualification
) AS TBL
ORDER BY Qualification

OUTPUT: 5         ORDER BY with CASE WHEN :


SELECT * FROM
(
 SELECT 'I.Sc.' AS Qualification
 UNION
 SELECT 'B.Sc.' AS Qualification
 UNION
 SELECT 'Other' AS Qualification
 UNION
 SELECT 'P.G.' AS Qualification
 UNION
 SELECT 'MCA' AS Qualification
 UNION
 SELECT 'DCA' AS Qualification
 UNION
 SELECT 'P.H.D' AS Qualification
) AS TBL
ORDER BY CASE WHEN Qualification='Other' 
THEN 'ZZZZZ' ELSE Qualification END

OUTPUT: 6           EXPLAINATION: Here we have set the value of item “Other” as “ZZZZZ” while the sorting is being done. Generally “ZZZZZ” will be the last item in any real world list and hence will move to the last. Scenario 2: In some other cases we may need the desired item to be moved to top of the result set. An example for this situation can be, a Dropdownlist containing items for Book Category. The list may have an item with text “General” , which is meant for those books whose category is not clearly known. Let’s assume that most of the books are supposed to be of this category. Then we have to place this item in the first index of the Dropdownlist so that it will be easier for user to pick the item. Now, we need a little modification in the ORDER BY clause as follows.


ORDER BY CASE WHEN Qualification='General' THEN NULL ELSE Qualification END

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