Understanding Joins
You combine data horizontally by merging or joining multiple
data sets into one data set. This process is called horizontal
combination because, in the final data set, each observation
(or horizontal row) has variables from more than one input data set.
Figure 3.1 PROC SQL: Understanding Joins
A PROC
SQL join is a query that specifies multiple tables or
views to be combined based on the conditions under which rows match
and return a result set.
Joins combine tables horizontally, side by side, by
combining rows. The tables being joined are not required to have the
same number of rows or columns.
Note: You can use a join to combine
views as well as tables. Most of the following references to tables
are also applicable to views; any exceptions are noted.
When you use a PROC
SQL query to join tables, you must decide how you want the rows from
the various tables to be combined. There are two main types of joins,
as shown below.
|
Type of Join
|
Output
|
|---|---|
|
Inner join
 |
only the rows that match
across all table or tables
|
|
Outer join
 |
rows that match across
tables (as in the inner join) plus nonmatching rows from one or more
tables
|
When any type of join
is processed, PROC SQL starts by generating a Cartesian product, which
contains all possible combinations of rows from all tables.
Last updated: October 16, 2019
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