12.4.2 Comparison Functions and Operators

Table 12.4 Comparison Operators

Name	Description
>	Greater than operator
>=	Greater than or equal operator
<	Less than operator
<>, !=	Not equal operator
<=	Less than or equal operator
<=>	NULL-safe equal to operator
=	Equal operator
BETWEEN ... AND ...	Whether a value is within a range of values
COALESCE()	Return the first non-NULL argument
EXISTS()	Whether the result of a query contains any rows
GREATEST()	Return the largest argument
IN()	Whether a value is within a set of values
INTERVAL()	Return the index of the argument that is less than the first argument
IS	Test a value against a boolean
IS NOT	Test a value against a boolean
IS NOT NULL	NOT NULL value test
IS NULL	NULL value test
ISNULL()	Test whether the argument is NULL
LEAST()	Return the smallest argument
LIKE	Simple pattern matching
NOT BETWEEN ... AND ...	Whether a value is not within a range of values
NOT EXISTS()	Whether the result of a query contains no rows
NOT IN()	Whether a value is not within a set of values
NOT LIKE	Negation of simple pattern matching
STRCMP()	Compare two strings

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.

The following relational comparison operators can be used to compare not only scalar operands, but row operands:

=  >  <  >=  <=  <>  !=

The descriptions for those operators later in this section detail how they work with row operands. For additional examples of row comparisons in the context of row subqueries, see Section 13.2.10.5, “Row Subqueries”.

Some of the functions in this section return values other than 1 (TRUE), 0 (FALSE), or NULL. LEAST() and GREATEST() are examples of such functions; Section 12.3, “Type Conversion in Expression Evaluation”, describes the rules for comparison operations performed by these and similar functions for determining their return values.

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 12.10, “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

  For row comparisons, (a, b) = (x, y) is equivalent to:

  (a = x) AND (b = y)

• <=>

  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.

  The <=> operator is equivalent to the standard SQL IS NOT DISTINCT FROM operator.

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

  For row comparisons, (a, b) <=> (x, y) is equivalent to:

  (a <=> x) AND (b <=> y)

• <>, !=

  Not equal:

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

  For row comparisons, (a, b) <> (x, y) and (a, b) != (x, y) are equivalent to:

  (a <> x) OR (b <> y)

• <=

  Less than or equal:

  mysql> SELECT 0.1 <= 2;
          -> 1

  For row comparisons, (a, b) <= (x, y) is equivalent to:

  (a < x) OR ((a = x) AND (b <= y))

• <

  Less than:

  mysql> SELECT 2 < 2;
          -> 0

  For row comparisons, (a, b) < (x, y) is equivalent to:

  (a < x) OR ((a = x) AND (b < y))

• >=

  Greater than or equal:

  mysql> SELECT 2 >= 2;
          -> 1

  For row comparisons, (a, b) >= (x, y) is equivalent to:

  (a > x) OR ((a = x) AND (b >= y))

• >

  Greater than:

  mysql> SELECT 2 > 2;
          -> 0

  For row comparisons, (a, b) > (x, y) is equivalent to:

  (a > x) OR ((a = x) AND (b > y))

• 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 12.3, “Type Conversion in Expression Evaluation”, but applied to all the three arguments.

  mysql> SELECT 2 BETWEEN 1 AND 3, 2 BETWEEN 3 and 1;
          -> 1, 0
  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

  For best results when using BETWEEN with date or time values, use CAST() to explicitly convert the values to the desired data type. Examples: If you compare a DATETIME to two DATE values, convert the DATE values to DATETIME values. If you use a string constant such as '2001-1-1' in a comparison to a DATE, cast the string to a DATE.

• 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.

  The return type of COALESCE() is the aggregated type of the argument types.

  mysql> SELECT COALESCE(NULL,1);
          -> 1
  mysql> SELECT COALESCE(NULL,NULL,NULL);
          -> NULL

• EXISTS(query)

  Whether the result of a query contains any rows.

  CREATE TABLE t (col VARCHAR(3));
  INSERT INTO t VALUES ('aaa', 'bbb', 'ccc', 'eee');
  
  SELECT EXISTS (SELECT * FROM t WHERE col LIKE 'c%');
          -> 1
  
  SELECT EXISTS (SELECT * FROM t WHERE col LIKE 'd%');
          -> 0

• NOT EXISTS(query)

  Whether the result of a query contains no rows:

  SELECT NOT EXISTS (SELECT * FROM t WHERE col LIKE 'c%');
          -> 0
  
  SELECT NOT EXISTS (SELECT * FROM t WHERE col LIKE 'd%');
          -> 1

• 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 (true) if expr is equal to any of the values in the IN() list, else returns 0 (false).

  Type conversion takes place according to the rules described in Section 12.3, “Type Conversion in Expression Evaluation”, applied to all the arguments. If no type conversion is needed for the values in the IN() list, they are all constants of the same type, and expr can be compared to each of them as a value of the same type (possibly after type conversion), an optimization takes place. The values the list are sorted and the search for expr is done using a binary search, which makes the IN() operation very quick.

  mysql> SELECT 2 IN (0,3,5,7);
          -> 0
  mysql> SELECT 'wefwf' IN ('wee','wefwf','weg');
          -> 1

  IN() can be used to compare row constructors:

  mysql> SELECT (3,4) IN ((1,2), (3,4));
          -> 1
  mysql> SELECT (3,4) IN ((1,2), (3,5));
          -> 0

  You should never mix quoted and unquoted values in an IN() list because the comparison rules for quoted values (such as strings) and unquoted values (such as numbers) differ. Mixing types may therefore lead to inconsistent results. For example, do not write an IN() expression like this:

  SELECT val1 FROM tbl1 WHERE val1 IN (1,2,'a');

  Instead, write it like this:

  SELECT val1 FROM tbl1 WHERE val1 IN ('1','2','a');

  Implicit type conversion may produce nonintuitive results:

  mysql> SELECT 'a' IN (0), 0 IN ('b');
          -> 1, 1

  In both cases, the comparison values are converted to floating-point values, yielding 0.0 in each case, and a comparison result of 1 (true).

  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 13.2.10.3, “Subqueries with ANY, IN, or SOME”.

• expr NOT IN (value,...)

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

• 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

• IS 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

• IS NOT boolean_value

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

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

• IS NULL

  Tests whether a value is NULL.

  mysql> SELECT 1 IS NULL, 0 IS NULL, NULL IS NULL;
          -> 0, 0, 1

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

  • If sql_auto_is_null variable is set to 1, then after a statement that successfully inserts an automatically generated AUTO_INCREMENT value, you can find that value by issuing a statement of the following form:

    SELECT * FROM tbl_name WHERE auto_col IS NULL

    If the statement returns a row, the value returned is the same as if you invoked the LAST_INSERT_ID() function. For details, including the return value after a multiple-row insert, see Section 12.15, “Information Functions”. If no AUTO_INCREMENT value was successfully inserted, the SELECT statement returns no row.

    The behavior of retrieving an AUTO_INCREMENT value by using an IS NULL comparison can be disabled by setting sql_auto_is_null = 0. See Section 5.1.7, “Server System Variables”.

    The default value of sql_auto_is_null is 0.

  • 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.

    See Obtaining Auto-Increment Values, and the description for the FLAG_AUTO_IS_NULL option at Connector/ODBC Connection Parameters.

• IS NOT NULL

  Tests whether a value is not NULL.

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

• 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 NULL.)

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

• LEAST(value1,value2,...)

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

  • If any argument is NULL, the result is NULL. No comparison is needed.

  • If all arguments are integer-valued, they are compared as integers.

  • If at least one argument is double precision, they are compared as double-precision values. Otherwise, if at least one argument is a DECIMAL value, they are compared as DECIMAL values.

  • If the arguments comprise a mix of numbers and strings, they are compared as numbers.

  • If any argument is a nonbinary (character) string, the arguments are compared as nonbinary strings.

  • In all other cases, the arguments are compared as binary strings.

  The return type of LEAST() is the aggregated type of the comparison argument types.

  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'