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12.1.3. Comparison Functions and Operators

Comparison operations result in a value of 1 (TRUE), 0 (FALSE), or NULL. These operations work for both numbers and strings. Strings are automatically converted to numbers and numbers to strings as necessary.

Some of the functions in this section (such as LEAST() and GREATEST()) return values other than 1 (TRUE), 0 (FALSE), or NULL. However, the value they return is based on comparison operations performed according to the rules described in Section 12.1.2, “Type Conversion in Expression Evaluation”.

To convert a value to a specific type for comparison purposes, you can use the CAST() function. String values can be converted to a different character set using CONVERT(). See Section 12.8, “Cast Functions and Operators”.

By default, string comparisons are not case sensitive and use the current character set. The default is latin1 (cp1252 West European), which also works well for English.

  • =

    Equal:

    mysql> SELECT 1 = 0;
            -> 0
    mysql> SELECT '0' = 0;
            -> 1
    mysql> SELECT '0.0' = 0;
            -> 1
    mysql> SELECT '0.01' = 0;
            -> 0
    mysql> SELECT '.01' = 0.01;
            -> 1
    
  • <=>

    NULL-safe equal. This operator performs an equality comparison like the = operator, but returns 1 rather than NULL if both operands are NULL, and 0 rather than NULL if one operand is NULL.

    mysql> SELECT 1 <=> 1, NULL <=> NULL, 1 <=> NULL;
            -> 1, 1, 0
    mysql> SELECT 1 = 1, NULL = NULL, 1 = NULL;
            -> 1, NULL, NULL
    
  • <>, !=

    Not equal:

    mysql> SELECT '.01' <> '0.01';
            -> 1
    mysql> SELECT .01 <> '0.01';
            -> 0
    mysql> SELECT 'zapp' <> 'zappp';
            -> 1
    
  • <=

    Less than or equal:

    mysql> SELECT 0.1 <= 2;
            -> 1
    
  • <

    Less than:

    mysql> SELECT 2 < 2;
            -> 0
    
  • >=

    Greater than or equal:

    mysql> SELECT 2 >= 2;
            -> 1
    
  • >

    Greater than:

    mysql> SELECT 2 > 2;
            -> 0
    
  • IS boolean_value, IS NOT boolean_value

    Tests a value against a boolean value, where boolean_value can be TRUE, FALSE, or UNKNOWN.

    mysql> SELECT 1 IS TRUE, 0 IS FALSE, NULL IS UNKNOWN;
            -> 1, 1, 1
    mysql> SELECT 1 IS NOT UNKNOWN, 0 IS NOT UNKNOWN, NULL IS NOT UNKNOWN;
            -> 1, 1, 0
    
  • IS NULL, IS NOT NULL

    Tests whether a value is or is not NULL.

    mysql> SELECT 1 IS NULL, 0 IS NULL, NULL IS NULL;
            -> 0, 0, 1
    mysql> SELECT 1 IS NOT NULL, 0 IS NOT NULL, NULL IS NOT NULL;
            -> 1, 1, 0
    

    To work well with ODBC programs, MySQL supports the following extra features when using IS NULL:

    • You can find the row that contains the most recent AUTO_INCREMENT value by issuing a statement of the following form immediately after generating the value:

      SELECT * FROM tbl_name WHERE auto_col IS NULL
      

      This behavior can be disabled by setting SQL_AUTO_IS_NULL=0. See Section 13.5.3, “SET Syntax”.

    • For DATE and DATETIME columns that are declared as NOT NULL, you can find the special date '0000-00-00' by using a statement like this:

      SELECT * FROM tbl_name WHERE date_column IS NULL
      

      This is needed to get some ODBC applications to work because ODBC does not support a '0000-00-00' date value.

  • expr BETWEEN min AND max

    If expr is greater than or equal to min and expr is less than or equal to max, BETWEEN returns 1, otherwise it returns 0. This is equivalent to the expression (min <= expr AND expr <= max) if all the arguments are of the same type. Otherwise type conversion takes place according to the rules described in Section 12.1.2, “Type Conversion in Expression Evaluation”, but applied to all the three arguments.

    mysql> SELECT 1 BETWEEN 2 AND 3;
            -> 0
    mysql> SELECT 'b' BETWEEN 'a' AND 'c';
            -> 1
    mysql> SELECT 2 BETWEEN 2 AND '3';
            -> 1
    mysql> SELECT 2 BETWEEN 2 AND 'x-3';
            -> 0
    
  • expr NOT BETWEEN min AND max

    This is the same as NOT (expr BETWEEN min AND max).

  • COALESCE(value,...)

    Returns the first non-NULL value in the list, or NULL if there are no non-NULL values.

    mysql> SELECT COALESCE(NULL,1);
            -> 1
    mysql> SELECT COALESCE(NULL,NULL,NULL);
            -> NULL
    
  • GREATEST(value1,value2,...)

    With two or more arguments, returns the largest (maximum-valued) argument. The arguments are compared using the same rules as for LEAST().

    mysql> SELECT GREATEST(2,0);
            -> 2
    mysql> SELECT GREATEST(34.0,3.0,5.0,767.0);
            -> 767.0
    mysql> SELECT GREATEST('B','A','C');
            -> 'C'
    

    GREATEST() returns NULL if any argument is NULL.

  • expr IN (value,...)

    Returns 1 if expr is equal to any of the values in the IN list, else returns 0. If all values are constants, they are evaluated according to the type of expr and sorted. The search for the item then is done using a binary search. This means IN is very quick if the IN value list consists entirely of constants. Otherwise, type conversion takes place according to the rules described in Section 12.1.2, “Type Conversion in Expression Evaluation”, but applied to all the arguments.

    mysql> SELECT 2 IN (0,3,5,'wefwf');
            -> 0
    mysql> SELECT 'wefwf' IN (0,3,5,'wefwf');
            -> 1
    

    The number of values in the IN list is only limited by the max_allowed_packet value.

    To comply with the SQL standard, IN returns NULL not only if the expression on the left hand side is NULL, but also if no match is found in the list and one of the expressions in the list is NULL.

    IN() syntax can also be used to write certain types of subqueries. See Section 13.2.8.3, “Subqueries with ANY, IN, and SOME.

  • expr NOT IN (value,...)

    This is the same as NOT (expr IN (value,...)).

  • ISNULL(expr)

    If expr is NULL, ISNULL() returns 1, otherwise it returns 0.

    mysql> SELECT ISNULL(1+1);
            -> 0
    mysql> SELECT ISNULL(1/0);
            -> 1
    

    ISNULL() can be used instead of = to test whether a value is NULL. (Comparing a value to NULL using = always yields false.)

    The ISNULL() function shares some special behaviors with the IS NULL comparison operator. See the description of IS NULL.

  • INTERVAL(N,N1,N2,N3,...)

    Returns 0 if N < N1, 1 if N < N2 and so on or -1 if N is NULL. All arguments are treated as integers. It is required that N1 < N2 < N3 < ... < Nn for this function to work correctly. This is because a binary search is used (very fast).

    mysql> SELECT INTERVAL(23, 1, 15, 17, 30, 44, 200);
            -> 3
    mysql> SELECT INTERVAL(10, 1, 10, 100, 1000);
            -> 2
    mysql> SELECT INTERVAL(22, 23, 30, 44, 200);
            -> 0
    
  • LEAST(value1,value2,...)

    With two or more arguments, returns the smallest (minimum-valued) argument. The arguments are compared using the following rules:

    • If the return value is used in an INTEGER context or all arguments are integer-valued, they are compared as integers.

    • If the return value is used in a REAL context or all arguments are real-valued, they are compared as reals.

    • If any argument is a case-sensitive string, the arguments are compared as case-sensitive strings.

    • In all other cases, the arguments are compared as case-insensitive strings.

    LEAST() returns NULL if any argument is NULL.

    mysql> SELECT LEAST(2,0);
            -> 0
    mysql> SELECT LEAST(34.0,3.0,5.0,767.0);
            -> 3.0
    mysql> SELECT LEAST('B','A','C');
            -> 'A'
    

    Note that the preceding conversion rules can produce strange results in some borderline cases:

    mysql> SELECT CAST(LEAST(3600, 9223372036854775808.0) as SIGNED);
            -> -9223372036854775808
    

    This happens because MySQL reads 9223372036854775808.0 in an integer context. The integer representation is not good enough to hold the value, so it wraps to a signed integer.


 
 
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