Beginning Microsoft SQL Server 2008 ... - S3 Tech Training

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This is supported across all versions and across most of the database management systems.
The UNION
OK, enough with all the “old syntax” versus “new syntax” stuff. Now we’re into something that’s the
same regardless of what other join syntax you prefer — the UNION operator. UNION is a special operator
we can use to cause two or more queries to generate one result set.
A UNION isn’t really a JOIN, like the previous options we’ve been looking at — instead it’s more of an
appending of the data from one query right onto the end of another query (functionally, it works a little
differently than this, but this is the easiest way to look at the concept). Where a JOIN combined information
horizontally (adding more columns), a UNION combines data vertically (adding more rows), as illustrated
in Figure 4-1.
When dealing with queries that use a UNION, there are just a few key points:
Chapter 4: JOINs
❑ All the UNIONed queries must have the same number of columns in the SELECT list. If your first
query has three columns in the SELECT list, then the second (and any subsequent queries being
UNIONed) must also have three columns. If the first has five, then the second must have five, too.
Regardless of how many columns are in the first query, there must be the same number in the
subsequent query(s).
❑ The headings returned for the combined result set will be taken only from the first of the
queries. If your first query has a SELECT list that looks like SELECT Col1, Col2 AS Second,
Col3 FROM..., then regardless of how your columns are named or aliased in the subsequent
queries, the headings on the columns returned from the UNION will be Col1, Second, and Col3
respectively.
❑ The data types of each column in a query must be implicitly compatible with the data type in
the same relative column in the other queries. Note that I’m not saying they have to be the same
data type — they just have to be implicitly convertible (a conversion table that shows implicit
versus explicit conversions can be found in Figure 1-3 of Chapter 1). If the second column in
the first query were of type char(20), then it would be fine if the second column in the second
query were varchar(50). However, because things are based on the first query, any rows
longer than 20 would be truncated for data from the second result set.
❑ Unlike non-UNION queries, the default return option for UNIONs is DISTINCT rather than ALL.
This can really be confusing to people. In your other queries, all rows were returned regardless
of whether they were duplicated with another row or not, but the results of a UNION do not work
that way. Unless you use the ALL keyword in your query, only one of any repeating rows will be
returned.
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