Testing for Nulls with IS NULL
Recall from “Nulls” in Chapter 3 that nulls represent missing or unknown values. This situation causes a problem: LIKE, BETWEEN, IN, and other WHERE clause options can’t find nulls because unknown values don’t satisfy specific conditions. Nulls match no value—not even other nulls.
In the table publishers, for example, note that publisher P03 has a null in the column state because that column doesn’t apply to Germany (Listing 4.42 and Figure 4.42). I can’t use complementary comparisons to select the null, because null is neither California nor not-California; it’s undefined (Listings 4.43 and 4.44, Figures 4.43 and 4.44).
To avert disaster, SQL provides IS NULL to determine whether a given value is null. The IS ...
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