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ClubA
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ClubC
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ClubB
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ClubD
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a) Union compatible
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Figure 7-4. Union compatiblity of tables
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b) Not union compatible
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Union
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Union allows you to combine all the rows in two union-compatible tables (or two sets of rows), as in the top left of Figure 7-2. Listing 7-1 shows the algebra expression for the union of the two tables ClubA and ClubB.
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Listing 7-1. Algebra Expresion for the Union of Two Compatible Tables ClubA ClubB
The order of the tables in the expression does not matter, because the resulting rows in the union will be the same; that is, ClubA ClubB = ClubB ClubA. To carry out a union in SQL, we need to first retrieve two sets of rows using two SELECT clauses. We can then combine the two sets with the UNION keyword. Listing 7-2 shows the SQL for performing a union between the pair of tables on the left side of Figure 7-4.
Listing 7-2. SQL for the Union of Two Compatible Tables SELECT * FROM ClubA UNION SELECT * FROM ClubB
CHAPTER 7 SE T OPERA TION S
The resulting table will include all the rows from both tables with no duplicates, so you will see only one row each for Barbara Olson and Robert Pollard, as shown in Figure 7-5. If you wish to retain the duplicates for some reason, you can use the key phrase UNION ALL.
Figure 7-5. Union of ClubA and ClubB
Union-compatible tables do not need to have the same column names. The names of the columns in the resulting virtual table will usually be from one of the tables. In the example in Figure 7-5, the column names are the same as the first table mentioned in the union query in Listing 7-2.
Ensuring Union Compatibility
When tables are not union compatible, you can often remedy the incompatibility in the SELECT clauses. For example, the two tables on the right side of Figure 7-4 have the columns in different orders. We can alter that order in the query, as shown in Listing 7-3.
Listing 7-3. Ensuring the Tables Have Columns in the Same Order SELECT MemberID, LastName, FirstName, Handicap, MemberType FROM ClubC UNION SELECT MemberID, LastName, FirstName, Handicap, MemberType FROM ClubD
Another incompatibility problem occurs when the types of the columns have been defined differently. For example, the ClubC table may have the Handicap field declared as an INT, whereas the ClubD table may have (unwisely) stored the Handicap values in a CHAR field. As I mentioned earlier, different implementations of SQL will treat these inconsistencies in a variety of ways. Many will try to convert the numbers to strings or vice versa. You can take control of these conversions yourself (which is probably a good idea) by using type-conversion functions.
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For example, in SQL Server, the expression CONVERT(INT, Handicap) would take the value in the Handicap field and convert it to an integer value. If the Handicap field in the ClubD table were a CHAR type, then Listing 7-4 would ensure that the types were integers in both tables. Of course, if any of the values in the ClubD table s Handicap column could not be converted to integers, you would get an error, and you would need to fix the data.
Listing 7-4. Ensuring the Tables Have Columns of the Same Type SELECT MemberID, LastName, FirstName, Handicap, MemberType FROM ClubC UNION SELECT MemberID, LastName, FirstName, CONVERT(INT, Handicap), MemberType FROM ClubD
Selecting the Appropriate Columns
When combining data from two tables, you need to think about what it is you actually want. The examples with the clubs are rather contrived (as you have no doubt noticed). It is very unlikely that two clubs would have members with the same ID numbers and identical membership types. For example, a more likely scenario is that if Barbara Olson did belong to two clubs, she would have different data in each club table. In the ClubA table, she might be a Senior with ID 258. In the ClubB table, she might be an Associate member with an ID of 4573. If we do the union in Listing 7-2, where we select all the columns from each table, the two rows for Barbara will be different, and so both will appear in the result of the union, as in Figure 7-6.
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