=

Equal:等于：

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:对于行比较，(a, b) = (x, y)相当于：

(a = x) AND (b = y)

<=>

NULL-safe equal. NULL安全等于。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.此运算符执行与=运算符类似的相等比较，但如果两个操作数都为NULL，则返回1而不是NULL；如果一个操作数为NULL，则返回0而不是NULL。

The <=> operator is equivalent to the standard SQL IS NOT DISTINCT FROM operator.<=>运算符等价于标准SQL的IS NOT DISTINCT FROM运算符。

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:对于行比较，(a, b) <=> (x, y)相当于：

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

<>, !=

Not equal:不等于：

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:对于行比较，(a, b) <> (x, y)和(a, b) != (x, y)相当于：

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

<=

Less than or equal:小于或等于：

mysql> SELECT 0.1 <= 2;
        -> 1

For row comparisons, (a, b) <= (x, y) is equivalent to:对于行比较，(a, b) <= (x, y)相当于：

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

<

Less than:小于：

mysql> SELECT 2 < 2;
        -> 0

For row comparisons, (a, b) < (x, y) is equivalent to:对于行比较，(a, b) < (x, y)相当于：

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

>=

Greater than or equal:大于或等于：

mysql> SELECT 2 >= 2;
        -> 1

For row comparisons, (a, b) >= (x, y) is equivalent to:对于行比较，(a, b) >= (x, y)相当于：

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

>

Greater than:大于：

mysql> SELECT 2 > 2;
        -> 0

For row comparisons, (a, b) > (x, y) is equivalent to:对于行比较，(a, b) > (x, y)相当于：

(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. 如果expr大于或等于min，expr小于或等于max，则BETWEEN返回1，否则返回0。This is equivalent to the expression (min <= expr AND expr <= max) if all the arguments are of the same type. 如果所有参数的类型相同，这相当于表达式(min <= expr AND expr <= max)。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.否则，类型转换将根据第12.3节，“表达式求值中的类型转换”中描述的规则进行，但适用于所有三个参数。

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. 为了在将BETWEEN与日期或时间值一起使用时获得最佳结果，请使用CAST()将值显式转换为所需的数据类型。Examples: If you compare a DATETIME to two DATE values, convert the DATE values to DATETIME values. 示例：如果将一个DATETIME与两个DATE值进行比较，请将DATE值转换为DATETIME值。If you use a string constant such as '2001-1-1' in a comparison to a DATE, cast the string to a DATE.如果使用字符串常量（如'2001-1-1'）比较DATE，请将字符串转换为DATE。

expr NOT BETWEEN min AND max

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

COALESCE(value,...)

Returns the first non-NULL value in the list, or NULL if there are no non-NULL values.返回列表中的第一个非NULL值，如果没有非NULL值，则返回NULL。

The return type of COALESCE() is the aggregated type of the argument types.COALESCE()的返回类型是参数类型的聚合类型。

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. 对于两个或多个参数，返回最大（最大值）参数。The arguments are compared using the same rules as for LEAST().使用与LEAST()相同的规则比较参数。

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.如果任一个参数为NULL，则GREATEST()返回NULL。

expr IN (value,...)

Returns 1 (true) if expr is equal to any of the values in the IN() list, else returns 0 (false).如果expr等于IN()列表中的任一个值，则返回1（true），否则返回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. 类型转换根据第12.3节，“表达式求值中的类型转换”中描述的规则进行，该规则适用于所有参数。If no type conversion is needed for the values in the IN() list, they are all non-JSON 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. 如果IN()列表中的值不需要类型转换，那么它们都是同一类型的非JSON常量，并且expr可以作为同一类型的值（可能在类型转换之后）与它们中的每一个进行比较，则会进行优化。The values the list are sorted and the search for expr is done using a binary search, which makes the IN() operation very quick.对列表中的值进行排序，并使用二进制搜索来搜索expr，这使得IN()操作非常快速。

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:IN()可用于比较行构造函数：

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. 不能在IN()列表中混合使用带引号和不带引号的值，因为带引号的值（如字符串）和不带引号的值（如数字）的比较规则不同。Mixing types may therefore lead to inconsistent results. 因此，混合类型可能导致不一致的结果。For example, do not write an IN() expression like this:例如，不要这样编写IN()表达式：

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

Instead, write it like this:需要改成这样：

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

Implicit type conversion may produce nonintuitive results:隐式类型转换可能会产生非直观的结果：

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).在这两种情况下，比较值都转换为浮点值，每种情况下的结果为0.0，比较结果为1（true）。

The number of values in the IN() list is only limited by the max_allowed_packet value.IN()列表中的值的数目仅受max_allowed_packet值的限制。

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.为了符合SQL标准，IN()不仅在左侧的表达式为NULL时返回NULL，而且在列表中找不到匹配项并且列表中的一个表达式为NULL时也返回NULL。

IN() syntax can also be used to write certain types of subqueries. IN()语法也可用于编写某些类型的子查询。See Section 13.2.11.3, “Subqueries with ANY, IN, or SOME”.请参阅第13.2.11.3节，“带有ANY、IN或SOME的子查询”。

expr NOT IN (value,...)

This is the same as NOT (expr IN (value,...)).这等同于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. 如果N < N1则返回0；如果N < N2则返回1，依此类推，如果N为NULL则返回-1。All arguments are treated as integers. 所有参数都被视为整数。It is required that N1 < N2 < N3 < ... < Nn for this function to work correctly. 为了使此功能正常工作，需要N1 < N2 < N3 < ... < Nn。This is because a binary search is used (very fast).这是因为使用了二进制搜索（非常快）。

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.根据布尔值测试值，其中boolean_value可以是TRUE、FALSE或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.根据布尔值测试值，其中boolean_value可以是TRUE、FLASE或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.测试值是否为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:为了更好地使用ODBC程序，MySQL在使用IS NULL时支持以下额外功能：

IS NOT NULL

Tests whether a value is not NULL.测试值是否为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.如果expr为NULL，则ISNULL()返回1，否则返回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. 可以使用ISNULL()代替=来测试值是否为NULL。(Comparing a value to NULL using = always yields NULL.)（将值与NULL进行比较时使用=始终会产生NULL。）

The ISNULL() function shares some special behaviors with the IS NULL comparison operator. ISNULL()函数与IS NULL比较运算符共享一些特殊行为。See the description of IS NULL.请参阅IS NULL的说明。

LEAST(value1,value2,...)

With two or more arguments, returns the smallest (minimum-valued) argument. 对于两个或多个参数，返回最小（最小值）参数。The arguments are compared using the following rules:使用以下规则比较参数：

The return type of LEAST() is the aggregated type of the comparison argument types.LEAST()的返回类型是比较参数类型的聚合类型。

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'