Comparison or Theta Operators
Dr. Codd introduced the term “theta operators” in his early papers, for what a programmer would have called a comparison predicate operator. The large number of data types in SQL makes doing comparisons a little harder than in other programming languages. Values of one data type have to be promoted to values of the other data type before the comparison can be done. SQL is a strongly typed language, which means a lot of type castings are not possible—you cannot turn Christmas into a number and find its square root.
The comparison operators are overloaded and will work for <numeric>, <character>, and <datetime> data types. The symbols and meanings for comparison operators are shown in Table 16.1.