All the queries looked at so far have been answered by accessing data from one table. Sometimes, however, answers to a query may require data from two or more tables.
For example, for the Corporate Librarian to display a list of
contributing authors with their long form department name requires
data from the
REPORT table (author) and data from the
(department name). Obtaining the data you need requires the ability
to combine two or more tables. This process is commonly referred to
as "joining the tables".
Two or more tables can be combined to form a single table by using the
join operation. The join operation is based on the premise that
there is a logical association between two tables based on a common
attribute that links the tables. Therefore, there must be a common
column in each table for a join operation to be executed. For
example, both the
REPORT table and the
DEPARTMENT table have the
department identification code in common. Thus, they can be joined.
Joining two tables in Texis is accomplished by using a
statement. The general form of the
SELECT statement when a join
operation is involved is:
SELECT column-name1 [,column-name2] ...
FROM table-name1, table-name2
WHERE table-name1.column-name = table-name2.column-name ;
The combination of table name with column name as stated in the
clause describes the Join condition.