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### 12.3.2 Comparison Functions and Operators

Table 12.3 Comparison Operators

NameDescription
BETWEEN ... AND ... Check whether a value is within a range of values
COALESCE() Return the first non-NULL argument
= Equal operator
<=> NULL-safe equal to operator
> Greater than operator
>= Greater than or equal operator
GREATEST() Return the largest argument
IN() Check 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
< Less than operator
<= Less than or equal operator
LIKE Simple pattern matching
NOT BETWEEN ... AND ... Check whether a value is not within a range of values
!=, <> Not equal operator
NOT IN() Check 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 13.2.9.5, “Row Subqueries”.

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 12.2, “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 12.10, “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))

• 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 boolean_value syntax was added in MySQL 5.0.2.

• 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 NOT boolean_value syntax was added in MySQL 5.0.2.

• 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 (the default), 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 12.13, “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 5.1.4, “Server System Variables”.

• 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.

• Tests whether a value is not NULL.

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

• 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 12.2, “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.

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

• 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

• 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'

Before MySQL 5.0.13, GREATEST() returns NULL only if all arguments are NULL. As of 5.0.13, it returns NULL if any argument is NULL.

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

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');

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

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

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

• 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.

• 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

• 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 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.

Before MySQL 5.0.13, LEAST() returns NULL only if all arguments are NULL. As of 5.0.13, it 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.