Corresponding columns in the Projection clauses for the
combined queries must have compatible data types, but the columns
do not need to use the same column names.
The following query
selects the state column from the customer table and
the corresponding code column from the state table. Figure 1: Query
SELECT DISTINCT state FROM customer
WHERE customer_num BETWEEN 120 AND 125
UNION
SELECT DISTINCT code FROM state
WHERE sname MATCHES '*a';
The query returns state code abbreviations
for customer numbers 120 through 125 and for states whose sname ends
in a. Figure 2: Query result
state
AK
AL
AZ
CA
DE
SD
VA
WV
In compound queries, the column names or display labels
in the first SELECT statement are the ones that appear in the results.
Thus, in the query, the column name state from the first SELECT
statement is used instead of the column name code from the
second.