The SQL Procedure

query-expression

Retrieves data from tables.

table-expression <set-operator table-expression>...

table-expression is described in table-expression.
set-operator is one of the following:

INTERSECT <CORRESPONDING> <ALL>

OUTER UNION <CORRESPONDING>

UNION <CORRESPONDING> <ALL>

EXCEPT <CORRESPONDING> <ALL>

Query Expressions and Table Expressions

A query-expression is one or more table-expressions. Multiple table expressions are linked by set operators. The following figure illustrates the relationship between table-expressions and query-expressions.

[IMAGE]

Set Operators

PROC SQL provides traditional set operators from relational algebra:

OUTER UNION: concatenates the query results.
UNION: produces all unique rows from both queries.
EXCEPT: produces rows that are part of the first query only.
INTERSECT: produces rows that are common to both query results.

A query-expression with set operators is evaluated as follows.

Each table-expression is evaluated to produce an (internal) intermediate result table.
Each intermediate result table then becomes an operand linked with a set operator to form an expression, for example, A UNION B.
If the query-expression involves more than two table-expressions, the result from the first two becomes an operand for the next set operator and operand, for example, (A UNION B) EXCEPT C, ((A UNION B) EXCEPT C) INTERSECT D, and so on.
Evaluating a query-expression produces a single output table.

Set operators follow this order of precedence unless they are overridden by parentheses in the expression(s): INTERSECT is evaluated first. OUTER UNION, UNION, and EXCEPT have the same level of precedence.

PROC SQL performs set operations even if the tables or views that are referred to in the table-expressions do not have the same number of columns. The reason for this is that the ANSI Standard for SQL requires that tables or views involved in a set operation have the same number of columns and that the columns have matching data types. If a set operation is performed on a table or view that has fewer columns than the one(s) with which it is being linked, PROC SQL extends the table or view with fewer columns by creating columns with missing values of the appropriate data type. This temporary alteration enables the set operation to be performed correctly.

CORRESPONDING (CORR) Keyword

The CORRESPONDING keyword is used only when a set operator is specified. CORR causes PROC SQL to match the columns in table-expressions by name and not by ordinal position. Columns that do not match by name are excluded from the result table, except for the OUTER UNION operator. See OUTER UNION.

For example, when performing a set operation on two table-expressions, PROC SQL matches the first specified column-name (listed in the SELECT clause) from one table-expression with the first specified column-name from the other. If CORR is omitted, PROC SQL matches the columns by ordinal position.

ALL Keyword

The set operators automatically eliminate duplicate rows from their output tables. The optional ALL keyword preserves the duplicate rows, reduces the execution by one step, and thereby improves the query-expression's performance. You use it when you want to display all the rows resulting from the table-expressions, rather than just the rows that are output because duplicates have been deleted. The ALL keyword is used only when a set operator is also specified.

OUTER UNION

Performing an OUTER UNION is very similar to performing the SAS DATA step with a SET statement. The OUTER UNION concatenates the intermediate results from the table-expressions. Thus, the result table for the query-expression contains all the rows produced by the first table-expression followed by all the rows produced by the second table-expression. Columns with the same name are in separate columns in the result table.

For example, the following query expression concatenates the ME1 and ME2 tables but does not overlay like-named columns. OUTER UNION of ME1 and ME2 Tables shows the result.

proc sql;
   title 'ME1 and ME2: OUTER UNION';
   select *
      from me1
   outer union
   select *
      from me2;

OUTER UNION of ME1 and ME2 Tables

To overlay columns with the same name, use the CORRESPONDING keyword.

proc sql;
   title 'ME1 and ME2: OUTER UNION CORRESPONDING';
   select *
      from me1
   outer union corr
   select *
      from me2;

In the resulting concatenated table, notice the following:

OUTER UNION CORRESPONDING retains all nonmatching columns.
For columns with the same name, if a value is missing from the result of the first table-expression, the value in that column from the second table-expression is inserted.
The ALL keyword is not used with OUTER UNION because this operator's default action is to include all rows in a result table. Thus, both rows from the table ME1 where IDnum is 1120 appear in the output.

UNION

The UNION operator produces a table that contains all the unique rows that result from both table-expressions. That is, the output table contains rows produced by the first table-expression, the second table-expression, or both.

Columns are appended by position in the tables, regardless of the column names. However, the data type of the corresponding columns must match or the union will not occur. PROC SQL issues a warning message and stops executing.

The names of the columns in the output table are the names of the columns from the first table-expression unless a column (such as an expression) has no name in the first table-expression. In such a case, the name of that column in the output table is the name of the respective column in the second table-expression.

In the following example, PROC SQL combines the two tables:

proc sql;
   title 'ME1 and ME2: UNION';
   select *
      from me1
   union
   select *
      from me2;

In the following example, ALL includes the duplicate row from ME1. In addition, ALL changes the sorting by specifying that PROC SQL make one pass only. Thus, the values from ME2 are simply appended to the values from ME1.

proc sql;
   title 'ME1 and ME2: UNION ALL';
   select *
      from me1
   union all
   select *
      from me2;

See Combining Two Tables for another example.

EXCEPT

The EXCEPT operator produces (from the first table-expression) an output table that has unique rows that are not in the second table-expression. If the intermediate result from the first table-expression has at least one occurrence of a row that is not in the intermediate result of the second table-expression, that row (from the first table-expression) is included in the result table.

In the following example, the IN_USA table contains flights to cities within and outside the USA. The OUT_USA table contains flights only to cities outside the USA. This example returns only the rows from IN_USA that are not also in OUT_USA:

proc sql;
    title 'Flights from IN_USA';
    select * from in_usa
    except
    select * from out_usa;

INTERSECT

The INTERSECT operator produces an output table that has rows that are common to both tables. For example, using the IN_USA and OUT_USA tables shown above, the following example returns rows that are in both tables:

proc sql;
    title 'Flights from IN_USA and OUT_USA';
    select * from in_usa
    intersect
    select * from out_usa;

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