<Source>.ReadPrevious (Function)

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Managing databases

HFSQL functions

Prefix syntax

ReadPrevious

Presentation

Example

Read operation according to a key item or not

Looping through records that match a condition

Locks

Memos

Miscellaneous

See also

<Source>.ReadPrevious (Function)

In french: <Source>.LitPrécédent

Not available with this kind of connection

Positions on the previous file record according to a browse item. The record is read and the HFSQL variables are updated (Customer.Name for example, which means the Name item of Customer file).

Values in the browse item are read in descending order (see remarks for more details).

Generally, <Source>.ReadPrevious sets the position in the data file during a loop operation.

Several cases may occur after the call to <Source>.ReadPrevious:

the data file is empty or no record corresponds to the filter (defined by <Source>.Filter): no reading is performed and <Source>.Out returns True.
the function tries to lock a record that is already locked in read-only: no read operation is performed, HErrorLock returns True and <Source>.Out returns True.
The management of locks is not available.
Access by JDBC: Locks cannot be managed in databases accessed by JDBC.

This function can be used with the data files, HFSQL views or queries.

Example

Customer.ReadLast(Name)
WHILE Customer.Out() = False
// Process the record
Customer.ReadPrevious(Name)
END

Syntax

<Result> = <Source>.ReadPrevious([<Browse item> [, <Options>]])

<Result>: Boolean

True if the previous record was read,
False if an error occurred (lock, end of data file, etc.): the record is not read. This problem can be caused by:
a positioning problem (empty file, ...): <Source>.Found returns False and HError returns 0.
an error: HError returns an integer other than 0. HErrorInfo returns more details.

<Source>: Type corresponding to the specified source

Name of data file, HFSQL view or query used.

<Browse item>: Optional character string

Name of item used to loop through the data file. If this name is not specified, <Source>.ReadPrevious will use:
For a data file: the last browse item used on this file by the last function for HFSQL management (starting with the letter H). If this item does not exist, the best browse item is automatically used.
For a query: the ORDER BY of query if it exists, otherwise the last item used.
For a view: the sort item of the view (if it exists), otherwise the last item used.

<Options>: Optional constant (or combination of constants)

Used to configure:
the lock set on the record read by <Source>.ReadPrevious.
the management of duplicates. By default, all the duplicates are browsed.
hDistinct If duplicates are found, this constant allows you to read a single record among the duplicates. This parameter is taken into account only if the browse is performed on a key item.
hLockNo No lock: the record can be read or modified by another application during the reading.
hLockReadWrite Lock in read/write: the record currently read cannot be read or modified by another application.
hLockWrite Lock in write mode: the record currently read can be read by another application but it cannot be modified by another application.

The lock options will have no effect if the locks are not supported by the OLE DB provider or by the Native Connector.
The lock options specified by <Source>.ReadPrevious will be ignored. The lock mode specified by <Source>.ReadFirst or <Source>.ReadLast will remain effective when <Source>.ReadPrevious and <Source>.ReadNext are called.
To change the lock mode, use:
<Source>.ReadSeekFirst or <Source>.ReadSeekLast,
<Source>.ReadFirst or <Source>.ReadLast.
<Source>.LockRecNum or <Source>.UnlockRecNum.
For Native Oracle Connector, a different lock mode can be specified for each record. However, if a transaction was started by SQLTransaction before setting the lock, the lock will only be released at the end of the transaction (SQLTransaction with the sqlCommit or sqlRollBack constant).
The lock options are ignored. Use locking read functions (HReadPreviousLock) kept for backward compatibility.
This parameter is ignored.
Access by JDBC: This parameter is ignored.

Remarks

Read operation according to a key item or not

If the item is a key, <Source>.ReadPrevious reads the record with the greatest key value. The sort order taken into account is the one specified in the analysis for this key.

If the item is not a key, <Source>.ReadPrevious reads the previous active record. When browsing the data file, the records will be sorted according to their record number.
In this case, the selected browse item will appear in red in the code editor and a warning will be displayed in the "Code" pane.

Remark: The automatic completion proposes the key items only.

Looping through records that match a condition

In most cases, <Source>.ReadSeek, <Source>.ReadSeekFirst and <Source>.ReadSeekLast are used to position in the data file to perform a browse loop among the records corresponding to a condition. <Source>.ReadNext and <Source>.ReadPrevious are used to read the next or previous record corresponding to the condition:

if the item is a key, <Source>.ReadPrevious reads the previous records corresponding to the values less than or equal to the sought value.
if the item used is not a key, <Source>.ReadPrevious will read the next records corresponding to the values equal to the sought value.

To ignore the search while going to the next or previous record, use one of the following functions:

Locks

By default (<Options> not specified), the record is not locked.

If a lock is erquested (hLockWrite or hLockReadWrite constant), the record will be read only if this record is not already locked.

The lock options will have no effect if the locks are not supported by the OLE DB provider or by the Native Connector.

Memos

The memos associated with the record can be automatically read (or not) when reading the record. <Source>.SetMemo is used to customize this automatic read operation.

If the memos are supported, the associated text memos are read when the record is read. The binary memos are read only when they are explicitly used (<Source>.ExtractMemo).

Miscellaneous

<Source>.RecNum returns the current record number.
<Source>.ChangeKey changes the search key while keeping the position on the current record.
To optimize the first iterations on a data file, use <Source>.Optimize on this data file.
This function replaces HReadPreviousLock and HReadPrevious, which were kept for compatibility with WINDEV 5.5.

Component: wd280hf.dll