This document covers the following topics:
Belongs to Function Group: Database Access and Update
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.
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
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table-expression | |||||||||
UNION |
[ALL ] [(]SELECT
selection
table-expression[)]
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EXCEPT |
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INTERSECT |
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ORDER
BY |
integer | ASC DESC
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column-reference | ||||||||||
expression | ||||||||||
statement
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END-SELECT
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(structured mode only) | |||||||||
LOOP |
(reporting mode only) |
Extended Set Syntax:
[WITH_CTE
common-table-expression,...]
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ORDER BY Clause | |||||||||||||||||||||||||||||||
[OPTIMIZE FOR integer
ROWS ]
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QUERYNO
integer |
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[WITH
HOLD ]
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[WITH
RETURN ]
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[IF-NO-RECORDS-FOUND-clause] | |||||||||||||||||||||||||||||||
statement
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For an explanation of the symbols used in the syntax diagram, see Syntax Symbols.
Syntax Element Description - Syntax 1:
Syntax Element | Description |
---|---|
INTO
|
INTO Clause:
The |
VIEW
|
VIEW Clause:
If one or more views are referenced in the |
table-expression
|
Table Expression:
A For further information, see table-expression. |
UNION
|
Query Involving UNION Clause:
|
ALL |
Including Redundant Rows:
Specifies the result set contains redundant (duplicate) rows. |
ORDER BY
|
ORDER BY Clause:
The |
IF NO RECORDS
FOUND
|
IF NO RECORDS FOUND Clause:
The |
END-SELECT
|
End of SELECT Statement:
In structured mode, the Natural reserved keyword
In reporting mode, the |
LOOP |
|
The following syntax elements belong to the SQL
Extended Set:
|
|
WITH_CTE
common-table-expression,... |
WITH_CTE Clause:
This optional clause allows you to define a result table which can
be referenced in any For further information, see WITH CTE common-table-expression,... below. |
OPTIMIZE FOR
|
OPTIMIZE FOR Clause:
This clause is only valid against DB2 databases. When used against other databases, it will cause runtime errors. |
WITH
CS/RS/UR/... |
WITH CS/RS/UR/... Clause:
This clause is only valid against DB2 databases. When used against other databases, it will cause runtime errors. |
QUERYNO
|
QUERYNO Clause:
The |
SKIP LOCKED DATA |
SKIP LOCKED DATA Clause:
The |
FETCH FIRST
|
FETCH FIRST Clause:
This clause is only valid against DB2 databases. When used against other databases, it will cause runtime errors. |
WITH HOLD
|
WITH HOLD Clause:
This clause is not currently supported. When used, it will cause a compiler error. |
WITH RETURN
|
WITH RETURN Clause:
This clause is not currently supported. When used, it will cause a compiler error. |
WITH ... SCROLL
|
WITH ... SCROLL Clause:
RDBMS scrollable cursors are enabled with this clause. Scrollable
cursors can be Note:
Scrollable cursors allow the application to position any row in the cursor at any time as long as the cursor is open. Scrollable cursors are not supported for Sybase databases at all. Scrollable cursors are not supported for the MS SQL Server DBLIB interface, but only for the MS SQL Server ODBC interface. The positioning is performed depending on the content of the
scroll_hv. The content is evaluated each time a
|
WITH ROWSET POSITIONING FOR …
ROWS |
WITH ROWSET POSITIONING FOR … ROWS
Clause:
This clause enables DB2 rowset processing, which corresponds to
Natural native DML multifetch processing. |
ROWS_RETURNED [:]
ret_row |
ROWS_RETURNED [:] ret_row
Clause:
This clause specifies an I4 variable which will receive the number of rows returned by DB2 on behalf of the last executed DB2 fetch operation for advanced multiple row processing. |
INTO
|
parameter, |
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.
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 {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:
Syntax Element | Description |
---|---|
parameter
|
If single parameters are specified as target fields, their number
and formats must correspond to the number and formats of the
Example: DEFINE DATA LOCAL 01 #NAME (A20) 01 #AGE (I2) END-DEFINE ... SELECT NAME, AGE INTO #NAME, #AGE FROM SQL-PERSONNEL ... The target fields |
correlation-name
|
If the Example: 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 ... |
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 |
integer | ASC |
|||||||
column-reference | DESC |
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 ...
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.
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:
[common-table-expression-name
[(column-name,…)]
AS (fullselect)
]
|
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 ]
|
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
|
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
|
|
|
ONLY
|
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_hv [GIVING
[:] sqlcode]
|
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.
Syntax Element | Description |
---|---|
ASENSITIVE
SCROLL |
ASENSITIVE scrollable cursors are
either INSENSITIVE - if the cursor is READ-ONLY - or
SENSITIVE DYNAMIC - if the cursor is not
READ-ONLY .
|
INSENSITIVE
SCROLL |
INSENSITIVE SCROLL refers to a
cursor that cannot be used in
Positioned
UPDATE or
Positioned
DELETE operations. In addition, once opened, an
INSENSITIVE SCROLL cursor does not reflect UPDATE ,
DELETE or INSERT operations against the base table
after the cursor was opened.
See also Note. |
SENSITIVE STATIC
SCROLL |
SENSITIVE STATIC SCROLL refers to a
cursor that can be used for Positioned UPDATE s or Positioned
DELETE operations. In addition, a SENSITIVE STATIC
SCROLL cursor reflects UPDATE and DELETE
operations of base table rows. The cursor does not reflect INSERT
operations.
See also Note. |
SENSITIVE DYNAMIC
SCROLL |
SENSITIVE DYNAMIC scrollable
cursors reflect UPDATE , DELETE and
INSERT operations against the base table while the cursor is
open.
|
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:
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.
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integer |
Option | Explanation |
---|---|
CURRENT |
Fetches the current row (again). |
FIRST |
Fetches the first row. |
LAST |
Fetches the last row. |
NEXT |
Fetches the row after the current one. This is the default value. |
PRIOR |
Fetch the row before the current one. |
+/-
integer |
Only applies in connection with Specifies the position of the row to be fetched
Enter a plus (+) or minus (-) sign followed by an integer. The default value is a plus (+). |
ABSOLUTE |
Only applies in connection with Uses |
RELATIVE
|
Only applies in connection with Uses |
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.
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:
SQLCODE | Explanation |
---|---|
0 |
FETCH operation successful, data
returned except for FETCH with option BEFORE or
AFTER .
|
+100 |
Row not found, cursor still open, no data returned. |
-1 |
General error while trying to
FETCH a row
|
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
.
Note:
This clause actually does not belong to Natural SQL; it represents
Natural functionality which has been made available to SQL loop
processing.
IF NO [RECORDS ] [FOUND ]
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||
ENTER
|
||
statement | ||
END-NOREC
|
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.
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.
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.
Common Set Syntax:
SELECT SINGLE
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[IF-NO-RECORDS-FOUND-clause] | |||||||||||||||||||||||||
statement
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Extended Set Syntax:
SELECT SINGLE
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[IF-NO-RECORDS-FOUND-clause] | ||||||||||||||||||||||||||||||||||||||||||||||||
statement
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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.
Syntax Element | Description | |
---|---|---|
INTO
|
INTO Clause:
The For further information and examples, see INTO Clause. |
|
VIEW
|
VIEW Clause:
If one or more views are referenced in the For further information and examples, see VIEW Clause. |
|
table-expression
|
Table Expression:
The For further information, see
|
|
WITH
CS/RR/UR
|
WITH - Isolation Level Clause:
This clause is only valid against DB2 databases. When used against other databases, it will cause runtime errors. |
|
CS |
Cursor Stability | |
RR |
Repeatable Read | |
UR |
Uncommitted Read | |
FETCH FIRST |
FETCH FIRST Clause:
The |
|
IF NO
RECORDS FOUND
|
IF NO RECORDS FOUND Clause:
This clause is used to initiate a processing loop if no records
meet the selection criteria specified in the preceding For further information, see IF NO RECORDS FOUND Clause. |
|
End of SELECT Statement:
In structured mode, the Natural reserved keyword
In reporting mode, the |
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.