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>9.1. Logical Operators</A
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><P
> The usual logical operators are available:
<P
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><TR
><TD
><TT
CLASS="LITERAL"
>AND</TT
></TD
></TR
><TR
><TD
><TT
CLASS="LITERAL"
>OR</TT
></TD
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><TT
CLASS="LITERAL"
>NOT</TT
></TD
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>
<ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> uses a three-valued logic system with true,
false, and <TT
CLASS="LITERAL"
>null</TT
>, which represents <SPAN
CLASS="QUOTE"
>"unknown"</SPAN
>.
Observe the following truth tables:
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><TT
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></TH
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><TT
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> AND <TT
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></TH
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></TT
> OR <TT
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></THEAD
><TBODY
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><TD
>TRUE</TD
><TD
>TRUE</TD
><TD
>TRUE</TD
><TD
>TRUE</TD
></TR
><TR
><TD
>TRUE</TD
><TD
>FALSE</TD
><TD
>FALSE</TD
><TD
>TRUE</TD
></TR
><TR
><TD
>TRUE</TD
><TD
>NULL</TD
><TD
>NULL</TD
><TD
>TRUE</TD
></TR
><TR
><TD
>FALSE</TD
><TD
>FALSE</TD
><TD
>FALSE</TD
><TD
>FALSE</TD
></TR
><TR
><TD
>FALSE</TD
><TD
>NULL</TD
><TD
>FALSE</TD
><TD
>NULL</TD
></TR
><TR
><TD
>NULL</TD
><TD
>NULL</TD
><TD
>NULL</TD
><TD
>NULL</TD
></TR
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>
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>TRUE</TD
><TD
>FALSE</TD
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>FALSE</TD
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>TRUE</TD
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><TD
>NULL</TD
><TD
>NULL</TD
></TR
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><P
></P
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>
</P
><P
> The operators <TT
CLASS="LITERAL"
>AND</TT
> and <TT
CLASS="LITERAL"
>OR</TT
> are
commutative, that is, you can switch the left and right operand
without affecting the result. But see <A
HREF="sql-expressions.html#SYNTAX-EXPRESS-EVAL"
>Section 4.2.14</A
> for more information about the
order of evaluation of subexpressions.
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