A lookup field is a read-only field that displays values at runtime based on search criteria you specify. In its simplest form, a lookup field is passed the name of an existing field to search on, a field value to search for, and a different field in a lookup dataset whose value it should display. 

For example, consider a mail-order application that enables an operator to use a lookup field to determine automatically the city and state that correspond to the zip code a customer provides. The column to search on might be called ZipTable.Zip, the value to search for is the customer's zip code as entered in Order.CustZip, and the values to return would be those for the ZipTable.City and ZipTable.State columns of the record where the value of ZipTable.Zip matches the current value in the Order.CustZip field.

1. Enter a name for the lookup field in the Name edit box. Do not enter the name of an existing field.
2. Choose a data type for the field from the Type combo box.
3. Enter the size of the field in the Size edit box, if appropriate. Size is only relevant for fields of type TStringField, TBytesField, and TVarBytesField.
4. Select Lookup in the Field type radio group. Selecting Lookup enables the Dataset and Key Fields combo boxes.
5. Choose from the Dataset combo box drop-down list the dataset in which to look up field values. The lookup dataset must be different from the dataset for the field component itself, or a circular reference exception is raised at runtime. Specifying a lookup dataset enables the Lookup Keys and Result Field combo boxes.
6. Choose from the Key Fields drop-down list a field in the current dataset for which to match values. To match more than one field, enter field names directly instead of choosing from the drop-down list. Separate multiple field names with semicolons. If you are using more than one field, you must use persistent field components.
7. Choose from the Lookup Keys drop-down list a field in the lookup dataset to match against the Source Fields field you specified in step 6. If you specified more than one key field, you must specify the same number of lookup keys. To specify more than one field, enter field names directly, separating multiple field names with semicolons.
8. Choose from the Result Field drop-down list a field in the lookup dataset to return as the value of the lookup field you are creating.

When you design and run your application, lookup field values are determined before calculated field values are calculated. You can base calculated fields on lookup fields, but you cannot base lookup fields on calculated fields. 

You can use the LookupCache property to hone the way lookup fields are determined. LookupCache determines whether the values of a lookup field are cached in memory when a dataset is first opened, or looked up dynamically every time the current record in the dataset changes. Set LookupCache to True to cache the values of a lookup field when the LookupDataSet is unlikely to change and the number of distinct lookup values is small. Caching lookup values can speed performance, because the lookup values for every set of LookupKeyFields values are preloaded when the DataSet is opened. When the current record in the DataSet changes, the field object can locate its Value in the cache, rather than accessing the LookupDataSet. This performance improvement is especially dramatic if the LookupDataSet is on a network where access is slow. 

If every record of DataSet has different values for KeyFields, the overhead of locating values in the cache can be greater than any performance benefit provided by the cache. The overhead of locating values in the cache increases with the number of distinct values that can be taken by KeyFields. 

If LookupDataSet is volatile, caching lookup values can lead to inaccurate results. Call RefreshLookupList to update the values in the lookup cache. RefreshLookupList regenerates the LookupList property, which contains the value of the LookupResultField for every set of LookupKeyFields values. 

When setting LookupCache at runtime, call RefreshLookupList to initialize the cache.