Select Expressions

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

• Table Expression

Selection

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

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

Syntax Element Description:

SELECT NAME, 65 - AGE 
  FROM SQL-PERSONNEL 
  ...

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

Table Expression

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

The following topics are covered below:

• FROM Clause

• Table Reference

• WHERE Clause

• GROUP BY Clause

• HAVING Clause

• ORDER BY Clause

• INPUT SEQUENCE

• ORDER OF table-designator

• FETCH FIRST Clause

• Examples of Table Expressions

FROM Clause

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

Table Reference

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.

TABLE function-name Clause

The TABLE function-name clause belongs to the SQL Extended Set and requires a correlation-clause with a column-name list.

Period Specification

The period-specification clause belongs to the SQL Extended Set.

FOR

SYSTEM_TIME BUSINESS_TIME

AS OF expr FROM expr1 TO expr2 BETWEEN expr1 AND expr2

period-specification optionally specifies that a period specification applies to the temporal table table-name. The same period name (SYSTEM_TIME or BUSINESS_TIME) must not be specified more than one time for the same table.

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

Correlation Clause

A correlation-clause consists of optional keyword AS and a correlation-name and is optionally followed by a plain column-name list. The column-name list belongs to the SQL Extended Set.

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:

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.

Data Change Table Reference

The data-change-table-reference clause belongs to the SQL Extended Set.

A data-change-table-reference specifies an intermediate result table, which is based on the rows that are changed by the SQL change statement specified in the clause. A data-change-table-reference can only be specified as the only table reference in the FROM clause.

Syntax Element Description:

XMLTABLE Function

The xmltable-function clause belongs to the SQL Extended Set.

The item xmltable-function specifies an invocation of the built-in XMLTABLE function.

WHERE Clause

The WHERE clause is used 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

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 
  ...

ORDER BY Clause

ORDER BY		sort-key		ASC DESC		,
		INPUT SEQUENCE
		ORDER OF table-designator

The order-by clause specifies row ordering of the result table.

sort-key

A sort-key-expression is an expression which is more than a column-name or an unsigned integer constant.

INPUT SEQUENCE

INPUT SEQUENCE belongs to the SQL Extended Set.

INPUT SEQUENCE indicates that the result table reflects the input order of the rows specified in the VALUES clause of an INSERT statement.

INPUT SEQUENCE can only be specified if an INSERT statement is specified in the from-clause.

ORDER OF table-designator

ORDER OF table-designator belongs to the SQL Extended Set.

ORDER OF specifies that the row ordering of the designated table by the table-designator is applied to the result table of the query.

table-designator uniquely identifies a base table, a view, or the nested table expression of a subselect.

FETCH FIRST Clause

The fetch-first clause belongs to the SQL Extended Set.

The fetch-first clause limits the number of rows that can be fetched. It improves the performance of queries when only a limited number of rows are needed.

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   
...