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### Combining and Negating Conditions with AND, OR, and NOT

Often, you’ll need to specify multiple conditions in a single WHERE clause—to, say, retrieve rows based on the values in multiple columns. You can use the AND and OR operators to combine two or more conditions into a compound condition. AND, OR, and a third operator, NOT, are logical operators. Logical operators, or Boolean operators, are operators designed to work with truth values: true, false, and unknown.

If you’ve programmed in other languages (or studied propositional logic), you’re familiar with the two-value logic (2VL) system. In two-value logic, the result of a logical expression is either true or false. 2VL assumes perfect knowledge, in which all propositions are known to be ...

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