Tables are what previous dBASE versions called database files , with the release of dBASE for Windows, the terminology has been changed to be consistent with that used in the general database community.
In past versions, memo fields were contained in a file that had the same base filename as the dBASE table with the extension. DBT , the field in the record held a byte pointer to the offset into the file of the portion associated with that record. Now however, an. MDX index key can be based on multiple fields, expressions, functions or combinations of these features.
The limitation is that the expression used can be a maximum of characters long and evaluate to an expression that is a maximum of characters. Indexes are created by clicking the index box in the Table Design Window or by selecting Table Table Utilities Manage Indexes from the main table menu.
An Expression Builder tool is available as with all operations that involve writing an expression that allows expressions to be written by selecting fields, expressions, functions and operators from list boxes; you can even include fields from other tables as part of the index key.
This feature is available through the Table Utilities menu. The Index box in the Table Design Window allows only the selection of ascending or descending order. A major advantage of using expressions or functions in an index key is to be able to use the UPPER function to create a 'dictionary order' that will index records based on the spelling of a character field that is case insensitive the UPPER function converts all characters to upper case.
This can be important if you want your table indexed on names in an area like San Antonio that has a large ethnic population where names frequently have mixed-case spellings. If you are using the American English language drivers set up through the IDAPI configuration at installation this situation can play havoc with trying to view records alphabetically since upper case comes before lower Zebra before aardvark.
Indexing does not effect the original table or the data in the fields, it adjusts the logical order in which the records are viewed. Sorting: Sorting a table results in a duplicate table being created with the records rearranged in the desired order.
Any updates performed on the sorted table will have no effect on the original table and vice verse. The Sort feature is accessed through the Table Utilities menu. Sort vs Index: Some considerations that should be taken into account when deciding if it would be better to sort or index the table are: Creating files An index file contains a list of records in a logical record order and their physical position in the table, the natural order of the table is left unchanged.
Sorting creates another table with the records physically rearranged in the sort order. The amount of available disk space can be an important consideration in this case. Processing Certain operations, such as searching for records or running a query, work faster when an index is used. Some operations, like linking tables requires an index. Using functions Expressions, functions and operators can be included as part of the key when building an index. Sorting requires only fields be used in the key.
Updating Adding records to an indexed table will automatically update the index. Adding records to a sorted table may require resorting if it is important to maintain the natural order of the records. Mixed Data Types Index keys must be all the same data type, otherwise it is necessary to use one of the various type conversion functions. Sorting can be done using data of different types.
Mixed Order In an index, the entire key expression is arranged in ascending or descending order. When sorting a table, each field used can be ordered separately. In general, sorting is better used with tables that are relatively static and are not frequently updated or when it is to be exported to another application. Tables that are used for data entry, updating, queries, forms or reports are more efficient when indexed.
The Browse mode is similar to the Browse mode of earlier versions. Data is laid out in tabular form like a spreadsheet. The number of records and the fields per record that are visible depend on the size of the window and font being used. The Columnar mode is equivalent to the Edit mode of previous versions with a single record displayed in a vertical column.
Switching between views is as simple as clicking a button on the button bar inset. Setting properties: Selecting Properties Table Records Window from the menu opens a dialog that allows you to set the properties of the fields, records or window.
By selecting the Fields tab, you can choose which fields will be displayed or hidden, add calculated fields or set individual properties of each field. The options available are: Heading Changes the displayed field heading. If the heading is not specified, the default is the field name. Template Controls the display and restricts input to specific characters. For example, adds dashes to phone numbers or zip codes.
When Specifies a condition that must be met before the field can be edited. Width Sets the column width of the field when viewed on the Browse mode. Read-only Prevents editing of the data in the field. Calculated fields are always read-only Valid Ensures that the value entered in the field is valid.
For example, to make sure that a zip code has at least five digits. Error Message Determines the message that is displayed when the Valid test fails.
Required In the Valid panel Setting this property prevents the user from leaving the field blank. Setting properties in the Record page lets you determine if editing will be allowed including adding and deleting records and selecting which records will be displayed. Moving to another record: The Table menu includes entries that allow movement forward or back one record, one page screen , to the first or last record, to a specific record number or to locate a record based on the contents of a field.
With the exception of location by record number, all of these feature are also available on the button bar. Soundex is a system of coding names that was developed for use in genealogical research that will find names with similar spellings, For example, when the Soundex feature is turned on select Sounds Like in the Find dialog box , a search for 'Smith' will also locate 'Smythe', 'Schmidt' and any other similar names.
Records can be viewed, entered, edited or added in any of the table views. Records can be deleted by checking the Delete check box in the browse mode or by selecting Delete Records from the Table Utilities menu. As with earlier versions of dBASE, deletion is a two step process. The step outlined above marks the record for deletion, actual removal of the record from the file is accomplished when the file is 'packed' by selecting Pack Records from the Table Utilities menu.
If you need to delete a record and add a new record at the same time, select Blank Selected Record from the Table menu. This feature will clear the data from all fields in the record without deleting the record leaving a blank record to enter the new data.
The availability of Windows features are also handy when it comes to editing data. This presents two problems: First, it does not permit using a field as a salutation field for mail merge, you do not want to send a letter that starts with 'Dear Dr Jim A'.
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