Select Expressions

SELECT selection   table-expression

A select-expression specifies a result table. It is used in the following Natural SQL statements: INSERT | SELECT | UPDATE

This document covers the following topics:

Selection

ALL
DISTINCT

scalar-expression [[AS] correlation-name],
*

A selection specifies the columns of the result set tables to be selected.

Syntax Element Description:

Syntax Element Description
ALL|DISTINCT
Elimination of Duplicate Rows:

Duplicate rows are not automatically eliminated from the result of a select-expression. To request this, specify the keyword DISTINCT.

The alternative to DISTINCT is ALL. ALL is assumed if neither is specified.

scalar-expression
Scalar Expression:

Instead of, or as well as, simple column names, a selection can also include general scalar expressions containing scalar operators and scalar functions which provide computed values (see also the section Scalar Expressions).

Example:

SELECT NAME, 65 - AGE 
  FROM SQL-PERSONNEL 
  ...
AS The optional keyword AS introduces a correlation-name for a column.
correlation-name
Correlation Name:

A correlation-name can be assigned to a scalar-expression as an alias name for a result column.

The correlation-name need not be unique. If no correlation-name is specified for a result column, the corresponding column-name will be used (if the result column is derived from a column name; if not, the result table will have no name). The name of a result column may be used, for example, as column name in the ORDER BY clause of a SELECT statement.

*
Asterisk Notation:

All columns of the result table are selected.

Example:

SELECT * 
  FROM SQL-PERSONNEL, SQL-AUTOMOBILES 
  ...

Table Expression

from-clause [where-clause]
[group-by-clause] [having-clause]

The table-expression specifies from where and according to what criteria rows are to be selected.

The following topics are covered below:

FROM Clause

FROM table-reference,…

This clause specifies from which tables the result set is built.

Table Reference

table-name [[AS] correlation-name]
subquery [AS] correlation-name
joined-table

The tables specified in the FROM clause must contain the column fields used in the selection list.

You can either specify a single table or produce an intermediate table resulting from a subquery or a "join" operation (see below).

Since various tables (that is, DDMs) can be addressed in one FROM clause and since a table-expression can contain several FROM clauses if subqueries are specified, the database ID (DBID) of the first DDM specified in the first FROM clause of the whole expression is used to identify the underlying database involved.

Optionally, a correlation-clause can be assigned to a table-name. For a subquery, a correlation-clause must be assigned.

Joined Table

 table-reference

INNER
LEFT [OUTER]
RIGHT [OUTER]
FULL [OUTER]

JOIN table-reference ON join-condition

(joined-table)

A joined-table specifies an intermediate table resulting from a "join" operation.

The "join" can be an INNER, LEFT OUTER, RIGHT OUTER or FULL OUTER JOIN. If you do not specify anything, INNER applies.

Multiple "join" operations can be nested; that is, the tables which create the intermediate result table can themselves be intermediate result tables of a "join" operation or a subquery; and the latter, in turn, can also have a joined-table or another subquery in its FROM clause.

Join Condition

For INNER, LEFT OUTER, and RIGHT OUTER joins:

search-condition

For FULL OUTER joins:

full-join-expression = full-join-expression [AND ]

Full Join Expression

 column-name

VALUE
COALESCE

(column-name , )

Within a join-expression only column-names and the scalar-function VALUE (or its synonym COALESCE) are allowed.

See details on column-name.

WHERE Clause

[WHERE search-condition]

The WHERE clause is used a to specify the selection criteria (search-condition) for the rows to be selected.

Example:

DEFINE DATA LOCAL 
01 NAME   (A20) 
01 AGE    (I2) 
END-DEFINE 
... 
SELECT * 
  INTO NAME, AGE 
  FROM SQL-PERSONNEL   
  WHERE AGE = 32 
END-SELECT 
...

For further information, see Search Conditions.

GROUP BY Clause

[GROUP BY column-reference,]

The GROUP BY clause rearranges the table represented by the FROM clause into groups in a way that all rows within each group have the same value for the GROUP BY columns.

Each column-reference in the selection list must be either a GROUP BY column or specified within an aggregate-function. Aggregate functions are applied to the individual groups (not to the entire table). The result table contains as many rows as groups.

For further information, see Column Reference and Aggregate Function.

Example:

DEFINE DATA LOCAL 
1 #AGE     (I2) 
1 #NUMBER  (I2) 
END-DEFINE 
... 
SELECT AGE , COUNT(*)  
  INTO #AGE, #NUMBER 
  FROM SQL-PERSONNEL 
  GROUP BY AGE 
  ...

If the GROUP BY clause is preceded by a WHERE clause, all rows that do not satisfy the WHERE clause are excluded before any grouping is done.

HAVING Clause

[HAVING search-condition]

If the HAVING clause is specified, the GROUP BY clause should also be specified.

Just as the WHERE clause is used to exclude rows from a result table, the HAVING clause is used to exclude groups and therefore also based on a search-condition. Scalar expressions in a HAVING clause must be single-valued per group.

For further information, see Scalar Expressions and Search Conditions.

Example:

DEFINE DATA LOCAL 
1 #NAME     (A20) 
1 #AVGAGE   (I2) 
1 #NUMBER   (I2) 
END-DEFINE 
... 
SELECT NAME, AVG(AGE), COUNT(*)  
  INTO #NAME, #AVGAGE, #NUMBER 
  FROM SQL-PERSONNEL 
  GROUP BY NAME 
  HAVING COUNT(*) > 1 
  ...

Examples of Table Expressions

Example 1:

DEFINE DATA LOCAL
01 #NAME      (A20)
01 #FIRSTNAME (A15)
01 #AGE       (I2)
...
END-DEFINE
...
SELECT NAME, FIRSTNAME, AGE
  INTO #NAME, #FIRSTNAME, #AGE
  FROM SQL-PERSONNEL
    WHERE NAME IS NOT NULL
      AND AGE > 20
...
  DISPLAY #NAME #FIRSTNAME #AGE   
END-SELECT
...
END

Example 2:

DEFINE DATA LOCAL
01 #COUNT    (I4)
...
END-DEFINE
...
SELECT SINGLE COUNT(*) INTO #COUNT FROM SQL-PERSONNEL   
...