Working with Recordsets
The Recordset object has built-in features that let you rearrange the order of the data in the result set, to search for a specific record based on criteria that you supply, and even to optimize those search operations using indexes. Whether these features are available for use depends on the provider and in some cases — such as that of the Index property — the structure of the data source itself.
Frequently, the most efficient way to order the data in your Recordset is by specifying an ORDER BY clause in the SQL command used to return results to it. However, you might have to change the order of the data in a Recordset that has already been created. You can use the Sort property to establish the order in which rows of a Recordset are traversed. Additionally, the Filter property determines which rows are can be accessed when traversing rows.
The Sort property sets or returns a String value that indicates the field names in the Recordset on which to sort. Each name is separated by a comma and is optionally followed by a space and the keyword ASC (which sorts the field in ascending order) or DESC (which sorts the field in descending order). By default, if no keyword is specified, the field is sorted in ascending order.
The sort operation is efficient because data is not physically rearranged but is accessed in the order specified by an index.
The Sort property requires the CursorLocation property to be set to adUseClient. A temporary index will be created for each field specified in the Sort property if an index does not already exist.
Setting the Sort property to an empty string will reset the rows to their original order and delete temporary indexes. Existing indexes will not be deleted.
Suppose a Recordset contains three fields named firstName, middleInitial, and lastName. Set the Sort property to the string "lastName DESC, firstName ASC", which will order the Recordset by last name in descending order and then by first name in ascending order. The middle initial is ignored.
No field referenced in a sort criteria string can be named "ASC" or "DESC" because those names conflict with the keywords ASC and DESC. Give a field that has a conflicting name an alias by using the AS keyword in the query that returns the Recordset.
For more information about Recordset filtering, see "Filtering the Results" later in this topic.
ADO provides the Find and Seek methods for locating a particular record in a Recordset. The Find method is supported by a variety of providers but is limited to a single search criterion. The Seek method supports searching on multiple criteria, but is not supported by many providers.
Indexes on fields can greatly enhance the performance of the Find method and Sort and Filter properties of the Recordset object. You can create an internal index for a Field object by setting its dynamic Optimize property. This dynamic property is added to the Properties collection of the Field object when you set the CursorLocation property to adUseClient. Remember that this index is internal to ADO — you cannot gain access to it or use it for any other purpose. Also, this index is distinct from the Index property of the Recordset object.
The Find method quickly locates a value within a column (field) of a Recordset. You can frequently improve the speed of the Find method on a column by using the Optimize property to create an index on it.
The Find method limits your search to the contents of one field. The Seek method requires that you have an index and has other limitations as well. If you must search on multiple fields that are not the basis of an index, or if your provider does not support indexes, you can limit your results by using the Filter property of the Recordset object.
The Find method searches a Recordset for the row that satisfies a specified criterion. Optionally, the direction of the search, starting row, and offset from the starting row may be specified. If the criterion is met, the current row position is set on the found record; otherwise, the position is set to the end (or start) of the Recordset, depending on search direction.
Only a single-column name can be specified for the criterion. In other words, this method does not support multicolumn searches.
The comparison operator for the criterion can be ">" (greater than), "<" (less than), "=" (equal), ">=" (greater than or equal), "<=" (less than or equal), "<>" (not equal), or "LIKE" (pattern matching).
The criterion value can be a string, floating-point number, or date. String values are delimited with single quotation marks or "#" (number sign) marks (for example, "state = 'WA'" or "state = #WA#"). Date values are delimited with "#" (number sign) marks (for example, "start_date > #7/22/97#").
If the comparison operator is "like", the string value can contain an asterisk (*) to find one or more occurrences of any character or substring. For example, "state like 'M*'" matches Maine and Massachusetts. You can also use leading and trailing asterisks to find a substring that is contained within the values. For example, "state like '*as*'" matches Alaska, Arkansas, and Massachusetts.
Asterisks can be used only at the end of a criteria string or together at both the beginning and end of a criteria string, as shown earlier. You cannot use the asterisk as a leading wildcard ('*str') or embedded wildcard ('s*r'). This will cause an error.
Seek and Index
Use the Seek method together with the Index property if the underlying provider supports indexes on the Recordset object. Use the Supports(adSeek) method to determine whether the underlying provider supports Seek, and the Supports(adIndex) method to determine whether the provider supports indexes. (For example, the OLE DB Provider for Microsoft Jet supports Seek and Index.)
If Seek does not find the desired row, no error occurs, and the row is positioned at the end of the Recordset. Set the Index property to the desired index before executing this method.
This method is supported only with server-side cursors. Seek is not supported when the CursorLocation property value of the Recordset object is adUseClient.
This method can be used only when the Recordset object has been opened with a CommandTypeEnum value of adCmdTableDirect.
The Find method limits your search to the contents of one field. The Seek method requires that you have an index and has other limitations as well. If you must search on multiple fields that are not the basis of an index or if your provider does not support indexes, you can limit your results by using the Filter property of the Recordset object.
Use the Filter property to selectively screen out records in a Recordset object. The filtered Recordset becomes the current cursor, which means that records that do not satisfy the Filter criteria are not available in the Recordset until the Filter is removed. Other properties that return values based on the current cursor are affected, such as AbsolutePosition, AbsolutePage, RecordCount, and PageCount. This is because setting the Filter property to a specific value will move the current record to the first record that satisfies the new value.
The Filter property takes a variant argument. This value represents one of three methods for using the Filter property: a criteria string, a FilterGroupEnum constant, or an array of bookmarks. For more information, see Filtering with a Criteria String, Filtering with a Constant, and Filtering with Bookmarks later in this topic.
When you know the data that you want to select, it is usually more efficient to open a Recordset with an SQL statement that effectively filters the result set, instead of relying on the Filter property.
To remove a filter from a Recordset, use the adFilterNone constant. Setting the Filter property to a zero-length string ("") has the same effect as using the adFilterNone constant.
Filtering with a Criteria String
The criteria string consists of clauses in the form FieldName Operator Value (for example, "LastName = 'Smith'"). You can create compound clauses by concatenating individual clauses with AND (for example, "LastName = 'Smith' AND FirstName = 'John'") and OR (for example, "LastName = 'Smith' OR LastName = 'Jones'"). Use the following guidelines for criteria strings:
FieldName must be a valid field name from the Recordset. If the field name contains spaces, you must enclose the name in square brackets.
Operator must be one of the following: <, >, <=, >=, <>, =, or LIKE.
Value is the value with which you will compare the field values (for example, 'Smith', #8/24/95#, 12.345, or $50.00). Use single quotation marks (') with strings and pound signs (#) with dates. For numbers, you can use decimal points, dollar signs, and scientific notation. If Operator is LIKE, Value can use wildcard characters. Only the asterisk (*) and percent sign (%) wildcard characters are allowed, and they must be the last character in the string. Value cannot be null.
To include single quotation marks (') in the filter Value, use two single quotation marks to represent one. For example, to filter on O'Malley, the criteria string should be "col1 = 'O''Malley'". To include single quotation marks at both the beginning and the end of the filter value, enclose the string in pound signs (#). For example, to filter on '1', the criteria string should be "col1 = #'1'#".
There is no precedence between AND and OR. Clauses can be grouped within parentheses. However, you cannot group clauses joined by an OR and then join the group to another clause with an AND, as follows.
(LastName = 'Smith' OR LastName = 'Jones') AND FirstName = 'John'
Instead, you would construct this filter as follows.
(LastName = 'Smith' AND FirstName = 'John') OR (LastName = 'Jones' AND FirstName = 'John')
In a LIKE clause, you can use a wildcard at the beginning and end of the pattern (for example, LastName Like '*mit*') or only at the end of the pattern (for example, LastName Like 'Smit*').
Filtering with a Constant
The following constants are available for filtering Recordsets.
Filters for viewing only records affected by the last Delete, Resync, UpdateBatch, or CancelBatch call.
Filters for viewing the records that failed the last batch update.
Filters for viewing the records in the current cache — that is, the results of the last call to retrieve records from the database.
Removes the current filter and restores all records for viewing.
Filters for viewing only records that have changed but have not yet been sent to the server. Applicable only for batch update mode.
The filter constants make it easier to resolve individual record conflicts during batch update mode by allowing you to view, for example, only those records that were affected during the last UpdateBatch method call, as shown in the following example.
Attribute VB_Name = "modExaminingData"
Filtering with Bookmarks
Finally, you can pass a variant array of bookmarks to the Filter property. The resulting cursor will contain only those records whose bookmark was passed in to the property. The following code example creates an array of bookmarks from the records in a Recordset which have a "B" in the ProductName field. It then passes the array to the Filter property and displays information about the resulting filtered Recordset.
'BeginFilterBkmk Dim vBkmkArray() As Variant Dim i As Integer 'Recordset created using "SELECT * FROM Products" as command. 'So, we will check to see if ProductName has a capital B, and 'if so, add to the array. i = 0 Do While Not objRs.EOF If InStr(1, objRs("ProductName"), "B") Then ReDim Preserve vBkmkArray(i) vBkmkArray(i) = objRs.Bookmark i = i + 1 Debug.Print objRs("ProductName") End If objRs.MoveNext Loop 'Filter using the array of bookmarks. objRs.Filter = vBkmkArray objRs.MoveFirst Do While Not objRs.EOF Debug.Print objRs("ProductName") objRs.MoveNext Loop 'EndFilterBkmk
Use the Clone method to create multiple, duplicate Recordset objects, especially if you want to maintain more than one current record in a given set of records. Using the Clone method is more efficient than creating and opening a new Recordset object with the same definition as the original.
The current record of a newly created clone is originally set to the first record. The current record pointer in a cloned Recordset is not synchronized with the original or vice versa. You can navigate independently in each Recordset.
Changes you make to one Recordset object are visible in all of its clones regardless of cursor type. However, after you execute Requery on the original Recordset, the clones will no longer be synchronized to the original.
Closing the original Recordset does not close its copies, nor does closing a copy close the original or any of the other copies.
You can clone a Recordset object only if it supports bookmarks. Bookmark values are interchangeable; that is, a bookmark reference from one Recordset object refers to the same record in any of its clones.