Question

Suppose you have these tables:

Table Name: Salesman
Fields: S_ID(Primary Key), Name

Table Name: Region_1
Fields: Reg_ID(Primary Key), S_ID(Foreign Key), sales

Table Name: Region_2
Fields: Reg_ID(Primary Key), S_ID(Foreign Key), sales

Table Name: Region_3
Fields: Reg_ID(Primary Key), S_ID(Foreign Key), sales

Table Name: Region_4
Fields: Reg_ID(Primary Key), S_ID(Foreign Key), sales

Query 1: Find out total of sales of each salesman in all the regions.

Query 2: Find out total of sales of a particual salesman in all the regions. (if the first one is solved I think this will be easy. 🙂 )

Answer 1

Query 1: Find out total of sales of each salesman in all the regions.

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Use:

   SELECT s.name,
          COALESCE(SUM(x.sales), 0) 
     FROM SALESMAN s
LEFT JOIN (SELECT r1.s_id,
                  r1.sales
             FROM REGION_1 r1
           UNION ALL
           SELECT r2.s_id,
                  r2.sales
             FROM REGION_2 r2
           UNION ALL
           SELECT r3.s_id,
                  r3.sales
             FROM REGION_3 r3
           UNION ALL
           SELECT r4.s_id,
                  r4.sales
             FROM REGION_4 r4) x ON x.s_id = s.s_id

Query 2: Find out total of sales of a particual salesman in all the regions.

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Add the WHERE clause to the query above:

   SELECT s.name,
          COALESCE(SUM(x.sales), 0) 
     FROM SALESMAN s
LEFT JOIN (SELECT r1.s_id,
                  r1.sales
             FROM REGION_1 r1
           UNION ALL
           SELECT r2.s_id,
                  r2.sales
             FROM REGION_2 r2
           UNION ALL
           SELECT r3.s_id,
                  r3.sales
             FROM REGION_3 r3
           UNION ALL
           SELECT r4.s_id,
                  r4.sales
             FROM REGION_4 r4) x ON x.s_id = s.s_id
    WHERE s.s_id = ?

…replace the ? with the salesperson’s s_id value.

The Lesson: UNION vs UNION ALL

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UNION and UNION ALL will allow you to combine two queries, but they have to have the same data types in the column positions. For example, say query one returns data types in the following order:

1. Integer
2. String

Any subsequent query that is union’d to the first has to return the same data types in the same position. Pay special attention to the fact of data types – it doesn’t guarantee the data within the column.

The second issue to UNIONs is knowing the difference between UNION and UNION ALL. UNION will remove duplicates if they exist (equivalent to using DISTINCT), which is not desired in the given scenario. UNION ALL will not remove duplicates, and is faster because of this.