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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="FUNCTIONS-COMPARISON"
>9.2. Comparison Operators</A
></H1
><P
>    The usual comparison operators are available, shown in <A
HREF="functions-comparison.html#FUNCTIONS-COMPARISON-TABLE"
>Table 9-1</A
>.
   </P
><DIV
CLASS="TABLE"
><A
NAME="FUNCTIONS-COMPARISON-TABLE"
></A
><P
><B
>Table 9-1. Comparison Operators</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><THEAD
><TR
><TH
>Operator</TH
><TH
>Description</TH
></TR
></THEAD
><TBODY
><TR
><TD
> <TT
CLASS="LITERAL"
>&lt;</TT
> </TD
><TD
>less than</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>&gt;</TT
> </TD
><TD
>greater than</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>&lt;=</TT
> </TD
><TD
>less than or equal to</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>&gt;=</TT
> </TD
><TD
>greater than or equal to</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>=</TT
> </TD
><TD
>equal</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>&lt;&gt;</TT
> or <TT
CLASS="LITERAL"
>!=</TT
> </TD
><TD
>not equal</TD
></TR
></TBODY
></TABLE
></DIV
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>     The <TT
CLASS="LITERAL"
>!=</TT
> operator is converted to
     <TT
CLASS="LITERAL"
>&lt;&gt;</TT
> in the parser stage.  It is not
     possible to implement <TT
CLASS="LITERAL"
>!=</TT
> and
     <TT
CLASS="LITERAL"
>&lt;&gt;</TT
> operators that do different things.
    </P
></BLOCKQUOTE
></DIV
><P
>    Comparison operators are available for all relevant data types.
    All comparison operators are binary operators that
    return values of type <TT
CLASS="TYPE"
>boolean</TT
>; expressions like
    <TT
CLASS="LITERAL"
>1 &lt; 2 &lt; 3</TT
> are not valid (because there is
    no <TT
CLASS="LITERAL"
>&lt;</TT
> operator to compare a Boolean value with
    <TT
CLASS="LITERAL"
>3</TT
>).
   </P
><P
>    
    In addition to the comparison operators, the special
    <TT
CLASS="TOKEN"
>BETWEEN</TT
> construct is available:
</P><PRE
CLASS="SYNOPSIS"
><TT
CLASS="REPLACEABLE"
><I
>a</I
></TT
> BETWEEN <TT
CLASS="REPLACEABLE"
><I
>x</I
></TT
> AND <TT
CLASS="REPLACEABLE"
><I
>y</I
></TT
></PRE
><P>
    is equivalent to
</P><PRE
CLASS="SYNOPSIS"
><TT
CLASS="REPLACEABLE"
><I
>a</I
></TT
> &gt;= <TT
CLASS="REPLACEABLE"
><I
>x</I
></TT
> AND <TT
CLASS="REPLACEABLE"
><I
>a</I
></TT
> &lt;= <TT
CLASS="REPLACEABLE"
><I
>y</I
></TT
></PRE
><P>
    Notice that <TT
CLASS="TOKEN"
>BETWEEN</TT
> treats the endpoint values as included
    in the range.
    <TT
CLASS="LITERAL"
>NOT BETWEEN</TT
> does the opposite comparison:
</P><PRE
CLASS="SYNOPSIS"
><TT
CLASS="REPLACEABLE"
><I
>a</I
></TT
> NOT BETWEEN <TT
CLASS="REPLACEABLE"
><I
>x</I
></TT
> AND <TT
CLASS="REPLACEABLE"
><I
>y</I
></TT
></PRE
><P>
    is equivalent to
</P><PRE
CLASS="SYNOPSIS"
><TT
CLASS="REPLACEABLE"
><I
>a</I
></TT
> &lt; <TT
CLASS="REPLACEABLE"
><I
>x</I
></TT
> OR <TT
CLASS="REPLACEABLE"
><I
>a</I
></TT
> &gt; <TT
CLASS="REPLACEABLE"
><I
>y</I
></TT
></PRE
><P>
    
    <TT
CLASS="LITERAL"
>BETWEEN SYMMETRIC</TT
> is the same as <TT
CLASS="LITERAL"
>BETWEEN</TT
>
    except there is no requirement that the argument to the left of
    <TT
CLASS="LITERAL"
>AND</TT
> be less than or equal to the argument on the right.
    If it is not, those two arguments are automatically swapped, so that
    a nonempty range is always implied.
   </P
><P
>    
    
    Ordinary comparison operators yield null (signifying <SPAN
CLASS="QUOTE"
>"unknown"</SPAN
>),
    not true or false, when either input is null.  For example,
    <TT
CLASS="LITERAL"
>7 = NULL</TT
> yields null, as does <TT
CLASS="LITERAL"
>7 &lt;&gt; NULL</TT
>.  When
    this behavior is not suitable, use the
    <TT
CLASS="LITERAL"
>IS [<SPAN
CLASS="OPTIONAL"
> NOT </SPAN
>] DISTINCT FROM</TT
> constructs:
</P><PRE
CLASS="SYNOPSIS"
><TT
CLASS="REPLACEABLE"
><I
>a</I
></TT
> IS DISTINCT FROM <TT
CLASS="REPLACEABLE"
><I
>b</I
></TT
>
<TT
CLASS="REPLACEABLE"
><I
>a</I
></TT
> IS NOT DISTINCT FROM <TT
CLASS="REPLACEABLE"
><I
>b</I
></TT
></PRE
><P>
    For non-null inputs, <TT
CLASS="LITERAL"
>IS DISTINCT FROM</TT
> is
    the same as the <TT
CLASS="LITERAL"
>&lt;&gt;</TT
> operator.  However, if both
    inputs are null it returns false, and if only one input is
    null it returns true.  Similarly, <TT
CLASS="LITERAL"
>IS NOT DISTINCT
    FROM</TT
> is identical to <TT
CLASS="LITERAL"
>=</TT
> for non-null
    inputs, but it returns true when both inputs are null, and false when only
    one input is null. Thus, these constructs effectively act as though null
    were a normal data value, rather than <SPAN
CLASS="QUOTE"
>"unknown"</SPAN
>.
   </P
><P
>    
    
    
    
    To check whether a value is or is not null, use the constructs:
</P><PRE
CLASS="SYNOPSIS"
><TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> IS NULL
<TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> IS NOT NULL</PRE
><P>
    or the equivalent, but nonstandard, constructs:
</P><PRE
CLASS="SYNOPSIS"
><TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> ISNULL
<TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> NOTNULL</PRE
><P>
    
   </P
><P
>    Do <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
> write
    <TT
CLASS="LITERAL"
><TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> = NULL</TT
>
    because <TT
CLASS="LITERAL"
>NULL</TT
> is not <SPAN
CLASS="QUOTE"
>"equal to"</SPAN
>
    <TT
CLASS="LITERAL"
>NULL</TT
>.  (The null value represents an unknown value,
    and it is not known whether two unknown values are equal.)
   </P
><DIV
CLASS="TIP"
><BLOCKQUOTE
CLASS="TIP"
><P
><B
>Tip: </B
>    Some applications might expect that
    <TT
CLASS="LITERAL"
><TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> = NULL</TT
>
    returns true if <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> evaluates to
    the null value.  It is highly recommended that these applications
    be modified to comply with the SQL standard. However, if that
    cannot be done the <A
HREF="runtime-config-compatible.html#GUC-TRANSFORM-NULL-EQUALS"
>transform_null_equals</A
>
    configuration variable is available. If it is enabled,
    <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> will convert <TT
CLASS="LITERAL"
>x =
    NULL</TT
> clauses to <TT
CLASS="LITERAL"
>x IS NULL</TT
>.
   </P
></BLOCKQUOTE
></DIV
><P
>    If the <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> is row-valued, then
    <TT
CLASS="LITERAL"
>IS NULL</TT
> is true when the row expression itself is null
    or when all the row's fields are null, while
    <TT
CLASS="LITERAL"
>IS NOT NULL</TT
> is true when the row expression itself is non-null
    and all the row's fields are non-null.  Because of this behavior,
    <TT
CLASS="LITERAL"
>IS NULL</TT
> and <TT
CLASS="LITERAL"
>IS NOT NULL</TT
> do not always return
    inverse results for row-valued expressions; in particular, a row-valued
    expression that contains both null and non-null fields will return false
    for both tests.  In some cases, it may be preferable to
    write <TT
CLASS="REPLACEABLE"
><I
>row</I
></TT
> <TT
CLASS="LITERAL"
>IS DISTINCT FROM NULL</TT
>
    or <TT
CLASS="REPLACEABLE"
><I
>row</I
></TT
> <TT
CLASS="LITERAL"
>IS NOT DISTINCT FROM NULL</TT
>,
    which will simply check whether the overall row value is null without any
    additional tests on the row fields.
   </P
><P
>    
    
    
    
    
    
    Boolean values can also be tested using the constructs
</P><PRE
CLASS="SYNOPSIS"
><TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> IS TRUE
<TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> IS NOT TRUE
<TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> IS FALSE
<TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> IS NOT FALSE
<TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> IS UNKNOWN
<TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> IS NOT UNKNOWN</PRE
><P>
    These will always return true or false, never a null value, even when the
    operand is null.
    A null input is treated as the logical value <SPAN
CLASS="QUOTE"
>"unknown"</SPAN
>.
    Notice that <TT
CLASS="LITERAL"
>IS UNKNOWN</TT
> and <TT
CLASS="LITERAL"
>IS NOT UNKNOWN</TT
> are
    effectively the same as <TT
CLASS="LITERAL"
>IS NULL</TT
> and
    <TT
CLASS="LITERAL"
>IS NOT NULL</TT
>, respectively, except that the input
    expression must be of Boolean type.
   </P
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