The usual comparison operators are available, as shown in Table 9.1.
Table 9.1. Comparison Operators
| Operator | Description | 
|---|---|
| datatype<datatype→boolean | Less than | 
| datatype>datatype→boolean | Greater than | 
| datatype<=datatype→boolean | Less than or equal to | 
| datatype>=datatype→boolean | Greater than or equal to | 
| datatype=datatype→boolean | Equal | 
| datatype<>datatype→boolean | Not equal | 
| datatype!=datatype→boolean | Not equal | 
     <> is the standard SQL notation for “not
     equal”.  != is an alias, which is converted
     to <> at a very early stage of parsing.
     Hence, it is not possible to implement !=
     and <> operators that do different things.
    
These comparison operators are available for all built-in data types that have a natural ordering, including numeric, string, and date/time types. In addition, arrays, composite types, and ranges can be compared if their component data types are comparable.
    It is usually possible to compare values of related data
    types as well; for example integer >
    bigint will work.  Some cases of this sort are implemented
    directly by “cross-type” comparison operators, but if no
    such operator is available, the parser will coerce the less-general type
    to the more-general type and apply the latter's comparison operator.
   
    As shown above, all comparison operators are binary operators that
    return values of type boolean.  Thus, expressions like
    1 < 2 < 3 are not valid (because there is
    no < operator to compare a Boolean value with
    3).  Use the BETWEEN predicates
    shown below to perform range tests.
   
There are also some comparison predicates, as shown in Table 9.2. These behave much like operators, but have special syntax mandated by the SQL standard.
Table 9.2. Comparison Predicates
| Predicate Description Example(s) | 
|---|
| 
         Between (inclusive of the range endpoints). 
         
         | 
| 
         
        Not between (the negation of  
         | 
| 
         Between, after sorting the two endpoint values. 
         | 
| 
         Not between, after sorting the two endpoint values. 
         | 
| 
         Not equal, treating null as a comparable value. 
         
         | 
| 
         Equal, treating null as a comparable value. 
         
         | 
| 
         Test whether value is null. 
         | 
| 
         Test whether value is not null. 
         | 
| 
         Test whether value is null (nonstandard syntax). | 
| 
         Test whether value is not null (nonstandard syntax). | 
| 
         Test whether boolean expression yields true. 
         
         | 
| 
         Test whether boolean expression yields false or unknown. 
         
         | 
| 
         Test whether boolean expression yields false. 
         
         | 
| 
         Test whether boolean expression yields true or unknown. 
         
         | 
| 
         Test whether boolean expression yields unknown. 
         
         | 
| 
         Test whether boolean expression yields true or false. 
         
         | 
    
    
    The BETWEEN predicate simplifies range tests:
aBETWEENxANDy
is equivalent to
a>=xANDa<=y
    Notice that BETWEEN treats the endpoint values as included
    in the range.
    BETWEEN SYMMETRIC is like BETWEEN
    except there is no requirement that the argument to the left of
    AND 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.
   
    The various variants of BETWEEN are implemented in
    terms of the ordinary comparison operators, and therefore will work for
    any data type(s) that can be compared.
   
     The use of AND in the BETWEEN
     syntax creates an ambiguity with the use of AND as a
     logical operator.  To resolve this, only a limited set of expression
     types are allowed as the second argument of a BETWEEN
     clause.  If you need to write a more complex sub-expression
     in BETWEEN, write parentheses around the
     sub-expression.
    
    
    
    Ordinary comparison operators yield null (signifying “unknown”),
    not true or false, when either input is null.  For example,
    7 = NULL yields null, as does 7 <> NULL.  When
    this behavior is not suitable, use the
    IS [ NOT ] DISTINCT FROM predicates:
aIS DISTINCT FROMbaIS NOT DISTINCT FROMb
    For non-null inputs, IS DISTINCT FROM is
    the same as the <> operator.  However, if both
    inputs are null it returns false, and if only one input is
    null it returns true.  Similarly, IS NOT DISTINCT
    FROM is identical to = for non-null
    inputs, but it returns true when both inputs are null, and false when only
    one input is null. Thus, these predicates effectively act as though null
    were a normal data value, rather than “unknown”.
   
To check whether a value is or is not null, use the predicates:
expressionIS NULLexpressionIS NOT NULL
or the equivalent, but nonstandard, predicates:
expressionISNULLexpressionNOTNULL
    Do not write
    expression = NULLNULL is not “equal to”
    NULL.  (The null value represents an unknown value,
    and it is not known whether two unknown values are equal.)
   
    Some applications might expect that
    expression = NULLexpression 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 transform_null_equals
    configuration variable is available. If it is enabled,
    PostgreSQL will convert x =
    NULL clauses to x IS NULL.
   
    If the expression is row-valued, then
    IS NULL is true when the row expression itself is null
    or when all the row's fields are null, while
    IS NOT NULL is true when the row expression itself is non-null
    and all the row's fields are non-null.  Because of this behavior,
    IS NULL and IS NOT NULL 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 row IS DISTINCT FROM NULL
    or row IS NOT DISTINCT FROM NULL,
    which will simply check whether the overall row value is null without any
    additional tests on the row fields.
   
Boolean values can also be tested using the predicates
boolean_expressionIS TRUEboolean_expressionIS NOT TRUEboolean_expressionIS FALSEboolean_expressionIS NOT FALSEboolean_expressionIS UNKNOWNboolean_expressionIS NOT UNKNOWN
    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 “unknown”.
    Notice that IS UNKNOWN and IS NOT UNKNOWN are
    effectively the same as IS NULL and
    IS NOT NULL, respectively, except that the input
    expression must be of Boolean type.
   
Some comparison-related functions are also available, as shown in Table 9.3.
Table 9.3. Comparison Functions