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>9.1. Logical Operators</A
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><P
>    The usual logical operators are available:

    

    

    

    

    

    

    <P
></P
><TABLE
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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
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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:

    <DIV
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><TH
><TT
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><I
>a</I
></TT
></TH
><TH
><TT
CLASS="REPLACEABLE"
><I
>b</I
></TT
></TH
><TH
><TT
CLASS="REPLACEABLE"
><I
>a</I
></TT
> AND <TT
CLASS="REPLACEABLE"
><I
>b</I
></TT
></TH
><TH
><TT
CLASS="REPLACEABLE"
><I
>a</I
></TT
> OR <TT
CLASS="REPLACEABLE"
><I
>b</I
></TT
></TH
></TR
></THEAD
><TBODY
><TR
><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
></TBODY
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><P
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>

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><TT
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></TH
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>NOT <TT
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><I
>a</I
></TT
></TH
></TR
></THEAD
><TBODY
><TR
><TD
>TRUE</TD
><TD
>FALSE</TD
></TR
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><TD
>FALSE</TD
><TD
>TRUE</TD
></TR
><TR
><TD
>NULL</TD
><TD
>NULL</TD
></TR
></TBODY
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><P
></P
></DIV
>
   </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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