SELECT - SQL

This document covers the following topics:

• Function

• Syntax 1 - Cursor-Oriented Selection

• Syntax 2 - Non-Cursor Selection

• Join Queries

Belongs to Function Group: Database Access and Update

Function

The SELECT statement supports both the cursor-oriented selection that is used to retrieve an arbitrary number of rows and the non-cursor selection (singleton SELECT) that retrieves at most one single row. With the SELECT ... END-SELECT construction, Natural uses the same database loop processing as with the FIND statement.

Two different structures are possible.

Syntax 1 - Cursor-Oriented Selection

Like the Natural FIND statement, the cursor-oriented SELECT statement is used to select a set of rows (records) from one or more DB2 tables, based on a search criterion. Since a database loop is initiated, the loop must be closed by a LOOP (reporting mode) or END-SELECT statement. With this construction, Natural uses the same loop processing as with the FIND statement.

In addition, no cursor management is required from the application program; it is automatically handled by Natural.

Common Set Syntax:

SELECT   selection INTO				parameter, VIEW {view-name [correlation-name]},		table-expression
		UNION		[ALL] [(]SELECT   selection table-expression[)]
		EXCEPT
		INTERSECT
ORDER BY				integer			ASC DESC
				column-reference
				expression
statement
END-SELECT		(structured mode only)
LOOP		(reporting mode only)

Extended Set Syntax:

[WITH_CTE common-table-expression,...]

SELECT selection  INTO   parameter, VIEW {view-name [correlation-name]}, table-expression

UNION EXCEPT INTERSECT [ALL] [(]SELECT selection table-expression[)]

ORDER BY Clause

[OPTIMIZE FOR integer ROWS]

WITH CS                                       RR UR RS RS KEEP UPDATE LOCKS RR KEEP UPDATE LOCKS

QUERYNO integer

FETCH FIRST 1 ROW ONLY                           integer ROWS

[WITH HOLD]

[WITH RETURN]

WITH ASENSITIVE SCROLL [:]scroll_hv [GIVING [:] sqlcode                       INSENSITIVE SCROLL SENSITIVE STATIC SCROLL SENSITIVE DYNAMIC SCROLL

WITH ROWSET POSITIONING FOR [:] row_hv ROWS ROWS_RETURNED [:] ret_row                       integer

[IF-NO-RECORDS-FOUND-clause]

statement

END-SELECT (structured mode only)                                               LOOP (reporting mode only)

For an explanation of the symbols used in the syntax diagram, see Syntax Symbols.

Syntax Element Description - Syntax 1:

INTO Clause

The INTO clause is used to specify the target fields in the program which are to be filled with the result of the selection.

The INTO clause can specify either single parameters or one or more views as defined in the DEFINE DATA statement.

All target field values can come either from a single table or from more than one table as a result of a join operation (see also Join Queries).

Note:
In standard SQL syntax, an INTO clause is only used in non-cursor select operations (singleton SELECT) and can be specified only if a single row is to be selected. In Natural, however, the INTO clause is used for both cursor-oriented and non-cursor select operations.

The selection can also merely consist of an asterisk (*). In a standard select expression, this is a shorthand for a list of all column names in the table(s) specified in the FROM clause. In the Natural SELECT statement, however, the same syntactical item SELECT * has a different semantic meaning: all the items listed in the INTO clause are also used in the selection. Their names must correspond to names of existing database columns.

Examples:

Example 1:

DEFINE DATA LOCAL
01 PERS VIEW OF SQL-PERSONNEL  
  02 NAME
  02 AGE
END-DEFINE
...
SELECT *
  INTO NAME, AGE

Example 2:

...
SELECT *
  INTO VIEW PERS

These examples are equivalent to the following ones:

Example 3:

...
SELECT NAME, AGE  
  INTO NAME, AGE

Example 4:

...
SELECT NAME, AGE  
  INTO VIEW PERS

VIEW Clause

VIEW {view-name [correlation-name]},

If one or more views are referenced in the INTO Clause, the number of items specified in the selection must correspond to the number of fields defined in the view(s) (not counting group fields, redefining fields and indicator fields).

Note:
Both the Natural target fields and the table columns must be defined in a Natural DDM. Their names, however, can be different, since assignment is made according to their sequence.

Example of INTO clause with view:

DEFINE DATA LOCAL
01 PERS VIEW OF SQL-PERSONNEL   
  02 NAME
  02 AGE
END-DEFINE
...
SELECT FIRSTNAME, AGE
  INTO VIEW PERS
  FROM SQL-PERSONNEL
...

The target fields NAME and AGE, which are part of a Natural view, receive the contents of the table columns FIRSTNAME and AGE.

Syntax Element Description:

DEFINE DATA LOCAL
01 #NAME   (A20)
01 #AGE    (I2)
END-DEFINE
...
SELECT NAME, AGE
  INTO #NAME, #AGE
  FROM SQL-PERSONNEL   
...

DEFINE DATA LOCAL
01 PERS VIEW OF SQL-PERSONNEL
  02 NAME
  02 FIRST-NAME
  02 AGE
END-DEFINE
...
SELECT *
  INTO VIEW PERS A
  FROM SQL-PERSONNEL A, SQL-PERSONNEL B  
...

Query Involving UNION

Note:
In the following, the term "SELECT statement" is used as a synonym for the whole query-expression consisting of multiple select expressions concatenated with UNION.

UNION unites the results of two or more select-expressions. The columns specified in the individual select-expressions must be UNION-compatible; that is, matching in number, type and format.

Redundant duplicate rows are always eliminated from the result of a UNION unless the UNION operator explicitly includes the ALL qualifier. With UNION, however, there is no explicit DISTINCT option as an alternative to ALL.

Example:

DEFINE DATA LOCAL
01 PERS VIEW OF SQL-PERSONNEL   
  02 NAME
  02 AGE
  02 ADDRESS (1:6)
END-DEFINE
...
SELECT NAME, AGE, ADDRESS
  INTO VIEW PERS
  FROM SQL-PERSONNEL
  WHERE AGE > 55
UNION ALL
SELECT NAME, AGE, ADDRESS
  FROM SQL-EMPLOYEES
  WHERE PERSNR < 100
ORDER BY NAME
...
END-SELECT
...

In general, any number of select-expressions can be concatenated with UNION.

The INTO clause must be specified with the first select-expression only.

ORDER BY Clause

The ORDER BY clause arranges the result of a SELECT statement in a particular sequence.

Each ORDER BY clause must specify a column of the result table. In most ORDER BY clauses a column can be identified either by column-reference (that is, by an optionally qualified column name) or by column number. In a query involving UNION, a column must be identified by column number. The column number is the ordinal left-to-right position of a column within the selection, which means it is an integer value. This feature makes it possible to order a result on the basis of a computed column which does not have a name.

Example:

DEFINE DATA LOCAL
1 #NAME          (A20)
1 #YEARS-TO-WORK (I2)
END-DEFINE
...
SELECT NAME , 65 - AGE
  INTO #NAME, #YEARS-TO-WORK   
  FROM SQL-PERSONNEL
  ORDER BY 2
  ...

Ascending or Descending Order

The order specified in the ORDER BY clause can be either ascending (ASC) or descending (DESC). ASC is the default.

Example:

DEFINE DATA LOCAL
1 PERS VIEW OF SQL-PERSONNEL
1 NAME
1 AGE
1 ADDRESS   (1:6)
END-DEFINE
...
SELECT NAME, AGE, ADDRESS
  INTO VIEW PERS
  FROM SQL-PERSONNEL
  WHERE AGE = 55
  ORDER BY NAME DESC
  ...

See further information on integer values and column-reference.

WITH_CTE common-table-expression,…

This clause permits to define result tables that can be referenced in any FROM clause of the SELECT statement that follows.

The Natural specific keyword WITH_CTE corresponds to the SQL keyword WITH. WITH_CTE will be translated into the SQL keyword WITH by the Natural compiler.

Each common-table-expression has to obey the following syntax:

Syntax Element Description:

A common-table-expression can be used

• in place of a view to avoid creating the view;

• when the same result table needs to be shared in a fullselect;

• when the result needs to be derived using recursion.

Queries using recursion are useful in applications such as bill of material.

Example:

WITH_CTE                                                        
 RPL (PART,SUBPART,QUANTITY) AS                                 
 (SELECT ROOT.PART,ROOT.SUBPART,ROOT.QUANTITY                   
    FROM HGK-PARTLIST ROOT                                      
   WHERE ROOT.PART ='01'                                       
  UNION ALL                                                     
  SELECT CHILD.PART,CHILD.SUBPART,CHILD.QUANTITY                
    FROM  RPL PARENT, HGK-PARTLIST CHILD                        
    WHERE PARENT.SUBPART = CHILD.PART                            
  )                                                             
SELECT DISTINCT PART,SUBPART,QUANTITY                           
  INTO VIEW V1                                                  
  FROM RPL                                                      
  ORDER BY PART,SUBPART,QUANTITY                                
END-SELECT

OPTIMIZE FOR integer ROWS Clause

The OPTIMIZE FOR integer ROWS clause is used to inform DB2 in advance of the number (integer) of rows to be retrieved from the result table. Without this clause, DB2 assumes that all rows of the result table are to be retrieved and optimizes accordingly.

This optional clause is useful if you know how many rows are likely to be selected, because optimizing for integer rows can improve performance if the number of rows actually selected does not exceed the integer value (which can be in the range from 0 to 2147483647).

Example:

SELECT name INTO 
#name FROM table WHERE AGE = 2 OPTIMIZE FOR 100 ROWS

WITH - Isolation Level Clause

This WITH clause allows you to specify an explicit isolation level with which the statement is to be executed. The following options are provided:

WITH UR can only be specified within a SELECT statement and when the table is read-only. The default isolation level is determined by the isolation of the package or plan into which the statement is bound. The default isolation level also depends on whether the result table is read-only or not. To find out the default isolation level, refer to the IBM literature.

Note:
This option also works for non-cursor selection.

FETCH FIRST Clause

The FETCH FIRST clause limits the number of rows to be fetched. It improves the performance of queries with potentially large result sets if only a limited number of rows is needed.

WITH ... SCROLL Clause

Natural supports SQL scrollable cursors by using the clauses WITH ASENSITIVE SCROLL, WITH SENSITIVE STATIC SCROLL and SENSITVE DYNAMIC SCROLL. Scrollable cursors allow Natural applications to position randomly any row in a result set. With non-scrollable cursors, the data can only be read sequentially, from top to bottom.

Note:
INSENSITIVE and SENSITIVE STATIC scrollable cursors use temporary result tables and require a TEMP database in DB2 (see the relevant DB2 literature by IBM).

Below is information on:

• scroll_hv
• scroll_hv Options
• GIVING [:] sqlcode

scroll_hv

The variable scroll_hv must be alphanumeric.

The variable scroll_hv specifies which row of the result table will be fetched during one execution of the database processing loop. The contents of scroll_hv is evaluated each time the database processing loop cycle is executed.

scroll_hv Options

There are some restrictions for special RDBMS systems:

• DB2 does not support the keyword CURRENT.

• In a SELECT FOR UPDATE loop DB2 only supports NEXT as scrolling option.

• MS SQL Server (ODBC interface) does not support the keyword CURRENT.

• Adabas D does not support RELATIVE scrolling.

GIVING [:] sqlcode

The specification of GIVING [:] sqlcode is optional. If specified, the Natural variable [:] sqlcode must be of format I4. The values for this variable are returned from the DB2 SQLCODE of the underlying FETCH operation. This allows the application to react to different statuses encountered while the scrollable cursor is open. The most important status codes indicated by SQLCODE are listed in the following table:

If you specify GIVING [:] sqlcode, the application must react to the different statuses. If an SQLCODE +100 is entered five times successively and without terminal I/O, the Natural for DB2 runtime will issue Natural error NAT3296 in order to avoid application looping. The application can terminate the processing loop by executing an ESCAPE statement.

If you do not specify GIVING [:] sqlcode, except for SQLCODE 0 and SQLCODE +100, each SQLCODE will generate Natural error NAT3700 and the processing loop will be terminated. SQLCODE +100 (row not found) will terminate the processing loop.

See also the example program DEM2SCRL supplied in the Natural system library SYSDB2.

IF NO RECORDS FOUND-Clause

Note:
This clause actually does not belong to Natural SQL; it represents Natural functionality which has been made available to SQL loop processing.

Structured Mode Syntax

IF NO [RECORDS] [FOUND]
	ENTER
	statement
END-NOREC

Reporting Mode Syntax

IF NO [RECORDS] [FOUND]
	ENTER
	statement
	DO statement DOEND

 The IF NO RECORDS FOUND clause is used to initiate a processing loop if no records meet the selection criteria specified in the preceding SELECT statement.

If no records meet the specified selection criteria, the IF NO RECORDS FOUND clause causes the processing loop to be executed once with an "empty" record. If this is not desired, specify the statement ESCAPE BOTTOM within the IF NO RECORDS FOUND clause.

If one or more statements are specified with the IF NO RECORDS FOUND clause, the statements are executed immediately before the processing loop is entered. If no statements are to be executed before entering the loop, the keyword ENTER must be used.

Note:
If the result set of the SELECT statement consists of a single row of NULL values, the IF NO RECORDS FOUND clause is not executed. This could occur if the selection list consists solely of one of the aggregate functions SUM, AVG, MIN or MAX on columns, and the set on which these aggregate functions operate is empty. When you use these aggregate functions in the above-mentioned way, you should therefore check the values of the corresponding null-indicator fields instead of using an IF NO RECORDS FOUND clause.

Database Values

Unless other value assignments are made in the statements accompanying an IF NO RECORDS FOUND clause, Natural resets to empty all database fields which reference the file specified in the current loop.

Evaluation of System Functions

Natural system functions are evaluated once for the empty record that is created for processing as a result of the IF NO RECORDS FOUND clause.

Syntax 2 - Non-Cursor Selection

Common Set Syntax:

SELECT SINGLE

selection  INTO parameter , table-expression       VIEW {view-name [correlation-name ]},

[IF-NO-RECORDS-FOUND-clause]

statement

END-SELECT (structured mode only)                                       LOOP (reporting mode only)

Extended Set Syntax:

SELECT SINGLE

selection  INTO parameter , table-expression       VIEW {view-name [correlation-name ]},

WITH CS                                 RR UR FETCH FIRST 1 ROW ONLY                     integer ROWS

[IF-NO-RECORDS-FOUND-clause]

statement

END-SELECT (structured mode only)                                       LOOP (reporting mode only)

For an explanation of the symbols used in the syntax diagram, see Syntax Symbols.

Syntax Element Description - Syntax 2:

The SELECT SINGLE statement supports the functionality of a non-cursor selection (singleton SELECT); that is, a select expression that retrieves at most one row without using a cursor. It cannot be referenced by a positioned UPDATE or a positioned DELETE statement.

Join Queries

A join is a query in which data is retrieved from more than one table. All the tables involved must be specified in the FROM clause.

Example:

DEFINE DATA LOCAL
1 #NAME     (A20)
1 #MONEY    (I4)
END-DEFINE
...
SELECT NAME, ACCOUNT
  INTO #NAME, #MONEY
  FROM  SQL-PERSONNEL P, SQL-FINANCE F   
  WHERE P.PERSNR = F.PERSNR
    AND F.ACCOUNT > 10000
    ...

A join always forms the Cartesian product of the tables listed in the FROM clause and later eliminates from this Cartesian product table all the rows that do not satisfy the join condition specified in the WHERE clause.

Correlation names can be used to save writing if table names are rather long. Correlation names must be used when a column specified in the selection list exists in more than one of the tables to be joined in order to know which of the identically named columns to select.