Pitney Bowes MapInfo Professional - Manual (Page 283)

When you choose Join in the Update Column for Thematic dialog box, you can see that MapInfo
Professional has already set up the Specify Join dialog box with the STATE fields from both tables.
If the join were not calculated automatically it would be necessary to specify the matching fields or
geographic join.
11. Choose OK.
MapInfo Professional calculates the sum and returns you to the Thematic - Step 2 of 3 dialog box.
The Field list box displays the temporary column you created: SumOfTot_pop.
However, before we create the map, we still need to calculate the percentage of the total population
of each state that is urban. That information does not exist as a field in the table, so you must build
an expression to generate it.
12. In the Field drop-down list, select Expression.
The Expression dialog box displays.
The expression SumOfTot_pop/Pop_1990 * 100 will give you the answer as a percent.
13. In the expression box, type:
SumOfTot_pop/Pop_1990*100
14. Click Verify to ensure that your syntax is correct, and then choose OK.
The Create Thematic - Step 2 of 3 dialog box shows the expression you created.
15. Click Next.
The Create Thematic Map - Step 3 of 3 wizard panel opens.
283 MapInfo Professional User Guide
Chapter 10: Creating Thematic and Other Themed Maps