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Text File | 1994-01-05 | 13.1 KB | 243 lines

Data Input Validation ===================== Type validation =============== By this term we mean the exercising of control over what characters a field will accept. RISC OS provides the means to allow writable icons to accept only certain characters, or ranges of characters, and reject others. The disallowed characters are simply ignored so that attempts to type them have no effect. If you open Powerbase's own application directory (SHIFT/double-click) you will find a file called 'ValStrings'. You are recommended not to alter this file unless you understand exactly what you are doing and, if you do so, to keep a copy of the original. Examining it, however, can give you an insight into the validation process. Look, for example, at the entry "03 Number". This refers to field type 3 (numeric). The line below is the validation string of the writable icon which makes up a numeric field. It looks like this:- Pptr_write,4,4;A0-9.+\- The intial 'P' is a command and signifies "pointer". 'ptr_write' is the name of the sprite used to represent the mouse pointer when it is over this type (and many other types) of field. You will have noticed that the usual RISC OS arrow changes to a blue vertical bar, somewhat resembling the caret, when over a writable field. '4,4' specifies the "active point" of the pointer. The next character is a semicolon which indicates that a new command follows. The 'A' which comes next is the command "allow" and is the part of the validation string which especially concerns us. It determines what characters the field will allow you to type. In this case they are the numerals 0-9, the decimal point and the plus and minus signs. Why the back- slash before the latter? Because '-' is a special character in an icon validation string (used, as in this case, to specify a range of characters) and so are the semicolon, tilde (~) and the backslash itself. To include any of these four characters in the validation string you must precede it with a backslash. A couple of further examples should be sufficient. "01 Alphanumeric" uses the same pointer type but the characters accepted are <space> (immediately after the 'A' for "allow"), the upper-case letters A-Z, the lower-case letters a-z, the numerals 0-9, some common punctuation and the "half" character. You might want to add to this list. "06 Calculated" uses a different pointer sprite (ptr_calc, which resembles a pocket calculator) and has no "allow" command at all: it doesn't need one because you can't type into this type of field, whose value is calculated from other fields. Table validation ================ This level of validation only comes into effect after entering data in a field which is linked to a validation table. Such fields are recognizable by having their text dark green instead of black. When you click the mouse over a new field or type RETURN, the linked validation table is scanned to see whether what you entered in the field occurs in the table. If it doesn't then Powerbase will restore the previous contents of the field and print an error message. Validation tables should be used whenever the contents of a field have to conform to certain "values". A manufacturing company will have a product code for every item which it makes and a database of customers which the firm supplies will make use of such codes to identify the items. It is essential that whoever enters the data is prevented from accidentally typing a spurious code. By linking the appropriate field to a validation table containing all the valid codes such mistakes can be prevented. Moreover, the validation table can also contain other data about the products, such as a name and a brief description, and lists can be printed in which this more understandable data appears instead of the cryptic codes. Up to nine items of data could be associated with each product code. The F.E. college database referred to in 'BrowseEdit' ("Using more than one subfile") makes extensive use of validation tables for subjects, courses, tutors and schools of origin. Not only does this trap typing errors but the data entry is greatly speeded up when one can type 'ELL' instead of 'English Language and Literature', yet have the latter printed out in full on a list or a timetable if one wishes. A further option with validation tables is to have Powerbase replace the typed-in code with a longer, more readable, form immediately on entry. Provided the substitute data will fit in the field the replacement occurs when you type RETURN or click the mouse in another field. If the replacement is too long then the typed-in data will be unchanged. The maximum number of validation tables is 20 and is determined by the text file 'Config' within the 'Resources' subdirectory in the Powerbase directory. Creating a validation table =========================== Choose 'Create table' from the 'Validation' submenu. Give the table a suitable name and enter the number of rows required. This will be the maximum number of values the table can contain. Always be generous in your estimates; tables can't be extended later. The next item required is the highest column number in the table. A maximum of 10 columns, with numbering from 0-9, is allowed. The most common situation is to have two columns. The first holds the short code which specifies what may be typed into the linked record field. The second is an expansion or explanation of the former. Define the maximum length of each field in the table and give each one a heading. Don't forget that the heading has to fit within whatever column width the field length will allow. There will only be single spaces between the columns. When you are satisfied with the data click 'OK' and the table will be created in memory. It is not saved on disc at this stage. You may enter data into it now or later. Entries may be freely altered and overwritten but do not forget to save the table whenever changes have been made. Linking tables to fields ======================== To link a validation table to a field the table must be in memory . Click MENU over the required data field and follow the 'Field' submenu, choosing the 'Link to table' option. A pair of "bump icons" permits you to cycle through the tables in memory. A menu, displayed by clicking SELECT or MENU over the right-pointing arrow to the right of the table name, provides an alternative way of choosing a table. When you have the name of the required one displayed choose the column of the table to which the field is to be linked. It is recommended that column 0 should normally provide the link (and therefore contain the data items which may be actually entered on the records) and this is set by default. You may, however, link to any column in the table and a second pair of "bump icons" allows you to cycle throught the column numbers. If you want the "replace on entry" option described previously then select the option switch for this. A third set of "bump icons" then becomes active, allowing you to choose which column of the table will replace the data which you type in. In such cases both the entries in the validation list proper and those in the replacement list are considered equally valid and either may be typed. e.g. in the case quoted above for the F.E. college database you could actually type in 'English Language and Literature' if you wished, instead of just 'ELL'. Powerbase would accept either, but the former will obviously take much longer to enter and you are far more likely to make a mistake (in which case Powerbase would object). Select the switch 'Link to table' and click on 'OK'. You will now see that the foreground colour of the linked field has changed from black to green. Place the caret in the field and click on 'List values' on the keypad. The validation table should be displayed. Displaying linked data ====================== The final entry on the 'Validation' submenu is 'Show relations'. Powerbase makes no claim to be anything but a flat-file database, but effective use of validation tables does give it some of the advantages normally to be found only in a relational database. If you choose the 'Show relations' entry (so that it becomes ticked) you will find that clicking the over a data field which is linked to a validation table causes a small window to appear to the right of the keypad. This window lists all the data which is on the same row of the validation table as the linked item. As you edit the record, moving from field to field, this window is constantly updated. It disappears when the caret is in a non-linked field. You can also bring up this window (whether 'Show relations' is ticked or not) by double-clicking with SELECT on a linked field - very handy if coded data is being used and you encounter a code you've never met before. Displaying validation tables ============================ When a table is linked to a field in the database record it may be displayed by placing the caret in the linked field and clicking on the keypad button 'List values' (keystroke equivalent f8). Alternatively, any table present in memory, whether linked to a data field or not, may be displayed by selecting it from the 'Display table' sub-menu (off the 'Validation' sub-menu). Clicking MENU over the table then offers a menu with entries enabling you to clear, save, print or sort the table. The validation table menu ========================= Clear ----- Removes all data from the table, leaving it blank. Since wiping out a table in this way is pretty drastic (and irrevocable) you will need 'Manager' level rights to do it, otherwise 'Clear' is shaded. Even so you are asked to confirm the operation before it actually takes place. When you close a database all the validation tables in memory are written to the disc so you will over-write your disc copy with the blank table. Save ---- This leads to a 'Save' box in which the default pathname points to a directory called 'ValTables' inside the database application directory. Since that's where Powerbase expects to find the tables for a database you should normally accept this pathname by clicking on 'OK' or typing RETURN. Only if you are transferring tables from one database to another should you need to drag the icon to a filer window. All validation tables in memory are, in any case, saved to the 'ValTables' directory when you close the database or quit Powerbase and you can also make this happen at any time by clicking on 'Force update' on the keypad. Print ----- Printing out the contents of a validation table (as opposed to printing a report containing data from a validation table) is done via this menu choice. The output appears in a window from which it may be saved as a text file (see 'Printing' - Output destination). Sort ---- This will sort the table on the first column. Loading validation tables ========================= Once a table is linked to a data field it will be automatically loaded when the database is opened. If not yet linked, however, the table won't be in memory unless you have just created it. To load the table choose 'Show table files' from the 'Validation' submenu. The contents of the 'ValTables' directory will be displayed and the required tables can dragged onto the record window (or simply double-clicked). As each table is dropped it will be displayed. N.B. Only one table at a time may be actually displayed. There may be times when the user wishes to use a validation table to hold some data, but doesn't wish to use it to validate input into a field or to link it to a field at all. How does one make sure the required table gets loaded? It is always possible to load a validation table once the database is open by using the procedure described in the previous paragraph, but there's a more elegant solution. By adding a "!" character to the end of the table's name you will cause Powerbase to load it whether any field is linked to it or not. Including validation table data in printouts ============================================ Validation tables are commonly used to allow short codes to be entered in records but with a link to a more descriptive entry in another column of the table. If this is all you require then always put the data which makes up the actual validation list, i.e. the items which are allowed in the fields of the main record, into the first column of the table and set the link from the record field to this column (column 0). The more detailed "expanded" entry should then go in column 1. Printing with the 'Expand' switch in the 'Print options' window selected will then print the column 1 instead of the column 0 entry. You can, however, include data from other columns of a table by displaying the table and clicking in the required columns with ADJUST, exactly like selecting main record fields for printing (see 'Printing'). It is immaterial which row you click on: only the column matters and the highlighting to show which columns are selected always appears in the first row. The columns selected in all tables are saved as part of a print selection and may therefore be retrieved for future use. As well as highlighting the required columns you will also need to select the 'Expand' switch as described above. Printed reports will then include all the data from the highlighted columns.