Check out example codes for "case when with count and combining similar values in sql". It will help you in understanding the concepts better.

Code Example 1

SELECT 
    data.id,
    data.lastname,
    data.firstname,
    SUM(data.WebData) AS WebData,
    SUM(data.InternalData) AS InternalData,
    SUM(data.countid) AS Countid
FROM
    (
       SELECT id,lastname,
              firstname,datasource,
              CASE WHEN Datasource = 'Web' THEN Count(Datasource) ELSE 0 END WebData,
              CASE WHEN Datasource = 'Internal' THEN Count(Datasource)ELSE 0 END InternalData,
             count(id) AS Countid

        FROM Table
        GROUP BY
                 id,lastname,firstname,datasource
     ) AS data
GROUP BY 
    data.id,data.firstname,data.lastname

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