HReadFirst (Function)

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HReadFirst (Function)

In french: HLitPremier

Not available with this kind of connection

Positions on the first 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 ascending order (for more details, see the remarks).

In most cases, HReadFirst is used to set the position in the data file in order to perform a read loop with HReadNext.

Several cases may occur after the call to HReadFirst:

the data file is empty or no record corresponds to the filter (defined by HFilter): no reading is performed and HOut returns True.
the function tries to read a record that is already locked in read mode: no read operation is performed, HErrorLock returns True and HOut returns True.

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

Example

// Read 1st record
HReadFirst(Customer, Name)
WHILE HOut(Customer) = False
// Process the record
HReadNext(Customer, Name)
END

Syntax

<Result> = HReadFirst([<Data file> [, <Browse item>] [, <Options>]])

<Result>: Boolean

Corresponds to:
False if an error occurs. In this case, HError returns an integer other than 0. HErrorInfo returns more details about the error. The record is not read.
the value of HFound in the other cases (the record can be read, even if <Result> returns False).

<Data file>: Optional character string

Name of data file, HFSQL view or query used. If this parameter is an empty string (""), HReadFirst manipulates the last data file used by the last HFSQL function (function starting with "H").

<Browse item>: Optional character string

Name of item used to loop through the data file or view (this parameter is ignored by the queries). If this name is not specified, HReadFirst 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

Used to configure:
the lock set on the record read by HReadFirst
whether the filter that was defined must be taken into account.
hForwardOnly
This constant can only be used with Native Connectors.
Optimizes simple iterations that do not use the following features:
Reading the previous record.
Modifying a record.
Saving position.
If one of these features is used, the result may differ from the expected one.
For example, this constant can be used to populate a Table control programmatically.
hKeepFilter The filter set by HFilter will be taken into account, even if the search key is not optimized for the filter. Reminder: HFilter returns the search key optimized for the filter.
Caution: in this case, performance issues may arise for large files.
hLockNo No lock: the record can be read or modified by another application during the reading.

hLockReadWrite Lock in read/write mode: 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.

hNoRefresh
HReadFirst does not refresh the content of the table or query. If possible, the query is not re-run. All the saved positions are stored.
The lock options will have no effect if the locks are not supported by the OLE DB provider or by the Native Connector.
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).

Remarks

Browse item

If the browse item is a key, HReadFirst reads the record with the lowest key value. The sort order taken into account is the one specified in the analysis for this key. If duplicates are found, HReadFirst reads the first "duplicate" record according to the sequence of record numbers.

If the browse item is not a key, HReadFirst reads the first active record. When browsing the 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.

Browsing queries

By default, HReadFirst re-executes the query to refresh the result. To avoid re-executing the query, we recommend that you use the hNoRefresh constant.

Browsing a query run with the hQueryWithoutCorrection constant:

To browse the records in the order returned by the database, there is no need to specify a browse item. Example:

HExecuteQuery(MyQuery, hQueryWithoutCorrection)
...
HReadFirst(MyQuery, hNoRefresh)

If a browse item is specified, the query result is retrieved and indexed in its entirety. The iteration is performed on the specified item. The initial sort of the query (specified by ORDER BY) is ignored. The created index (in HFSQL format) is sensitive to the case, to the punctuation, to the accented characters and in ascending order.

Example:

HExecuteQuery(MyQuery, hQueryWithoutCorrection)
...
HReadFirst(MyQuery, MyItem, hNoRefresh)

The created index is used to perform searches on the query result.

Locks

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

If a lock is requested (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. HSetMemo 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 (HExtractMemo).

Password

If HReadFirst is the first function that handles the specified data file, the password is checked when the opening data file. If the password is incorrect, HErrorPassword returns True and HOut returns True.

Native XML Connector

The behavior of HReadFirst depends on HActivateAutoFilter/HDeactivateAutoFilter.

HActivateAutoFilter is enabled by default.

Therefore, to read the content of XML file, read the content of main file (the parent) then read the content of linked files (the children).

When reading a data file, a filter is automatically applied to the linked data files in order to only read the records corresponding to the main file.

For example:

The email of this person can be retrieved when browsing the Person file.

To do so, simply set the position on the "Person" file and apply HReadFirst to the "Email" file.

In this case, the record read in the "Email" file will correspond to the first email associated with the current record in the "Person" file.

If this mechanism is disabled (HDeactivateAutoFilter), the record read in the "Email" file will correspond to the first record found in the Email file (and not to the child of the record read in the "Person" file).

Miscellaneous

HRecNum returns the current record number.
HChangeKey changes the search key while keeping the position on the current record.
To improve the first browse times on a file, use HOptimize on this data file.
This function replaces HReadFirstLock and HReadFirstNoLock, which were kept for compatibility with WINDEV 5.5.

Component: wd300hf.dll