Xbasic

EXIST Function

Syntax

Key_Exist_Flag as L = EXIST(A keyvalue,C tablename,C tagname)

Arguments

keyvalue

A string constant, character variable, or an expression that returns a character value. Any type

tablename

The filename of the table you want to search. The Table_Name parameter can include a full drive and path specification. If you omit the drive and path, Alpha Anywhere searches the current directory.

tagname

The name of an index that may contain the Key_Value. The Index_Name parameter can include a full drive and path specification. If you omit the drive and path, Alpha Anywhere searches the current directory.

Description

Returns TRUE if the specified file exists.

Discussion

EXIST() returns .T. (TRUE) if the Key_Value exists in the Index_Name index; otherwise, it returns .F. (FALSE). The EXIST() function is useful for checking if a record with a certain Key_Value exists in a table. You can use this function to prevent duplicate records from being entered into a table, or to ensure that a similar record exists in another table before you can enter the record into the current table. If you want to check for numeric or date values within a table, we recommend that you specify an index that converts the values to the character type. The following is an example Index expression for a numeric field containing dollar values:

ltrim( str(RETAIL, 10, 2) )

LTRIM()trims the leading blank spaces so that you can specify a number like "50.15" as a key value and the STR()function converts values in the RETAIL numeric field to character. The following is an example Index expression for a date field:

dtoc(INV_DATE)

You can then specify a date as the key value in the MM/DD/YYYY format. Or use the CDATE() function in place of DTOC().

Examples 

Assume that the following table contains the records shown, and that the index PROD_ORD is defined as TRIM(PROD_NAME):

PROD_ID
PROD_NAME
P005

Silver Fox Golf Woods

P006

Silver Fox Irons

EXIST("silver fox irons", "PRODUCT.DBF", "PROD_ORD") ? TRUE for the second record.

Assume that you are entering names and addresses into a contact table ( CONTACTS ), and want to prevent the entry of duplicate records. A duplicate record has the same values in the FIRSTNAME, LASTNAME, and COMPANY fields. During data entry, the prompt order forces you to enter data into the FIRSTNAME field first, then the LASTNAME and COMPANY fields. After you enter a value in the COMPANY field, check to see if the record is a duplicate. In addition, you only want the validation rule to fire during data entry, not when you edit an existing record. You therefore need to incorporate a some Xbasic methods into the expression that tell you the table's mode (0 = "View", 1 = "Change" and "2" = "Enter"). To do this:

1. Create an index in the CONTACTS table named CONTACT_ID using the following index expression:

trim(FIRSTNAME) + trim(LASTNAME) + COMPANY

2. Define the following expression as the validation rule for the COMPANY field:

iif(table.current().mode_get() = 2, exist(trim(FIRSTNAME) + trim(LASTNAME) + COMPANY, "CONTACTS", "CONTACT_ID"), .T.)

See Also