Now consider examples 9 and 10, that are a bit more complex than 1 through 8:
- POP_1990 > POP_1980*1.2
- round((POP_1980*1.2)/TOTAL_AREA,.1)
We created expression 9 by taking expression 3 and replacing the constant "17893" with the whole of expression 6. Similarly, we created expression 10 by taking expression 8 and replacing "POP_1990" with expression 6.
In general, you create complex expressions by combining simple expressions. Perhaps the best way to learn how to do this is look at some of the examples we have provided (above and following) and create your own expressions based on those examples. However, there are two very general techniques for creating complex expressions.
- Replace a column name or a specific value with a simple expression.
- Combine expressions using logical operators (and, not, or).
Example six illustrates the first technique. There are many examples of the second technique throughout this entry. See particularly the discussion following Using Logical Operators in Expressions.