MySQL 5.7 Reference Manual Chapter 13 Functions and Operators 参考手册第十三章函数与操作符内容总结

默认函数名和左括号之间不能存在空格，用来帮助 MySQL 解释器区分函数调用和表名列名引用

为了简洁表示，接下来所有的示例将采用简短输出

mysql> SELECT MOD(29,9);
+-----------+
| mod(29,9) |
+-----------+
|         2 |
+-----------+
1 rows in set (0.00 sec)

简化如下输出

mysql> SELECT MOD(29,9);
        -> 2

操作符操作不同类型，将发生类型转化，有时会隐式转化

mysql> SELECT 1+'1';
        -> 2
mysql> SELECT CONCAT(2,' test');
        -> '2 test'

显式转换数字至字符串用 CAST() 函数，通过 CONCAT() 函数将会隐式转换，因为需要字符串参数

mysql> SELECT 38.8, CAST(38.8 AS CHAR);
        -> 38.8, '38.8'
mysql> SELECT 38.8, CONCAT(38.8);
        -> 38.8, '38.8'

mysql> SELECT 1 > '6x';
        -> 0
mysql> SELECT 7 > '6x';
        -> 1
mysql> SELECT 0 > 'x6';
        -> 0
mysql> SELECT 0 = 'x6';
        -> 1

用数字查询字符串列，MySQL 无法使用索引，因为很多不同的字符串可以转换成该数字，譬如 '1', ' 1', or '1a'
SELECT * FROM tbl_name WHERE str_col=1;

mysql> SELECT '18015376320243458' = 18015376320243458;
        -> 1
mysql> SELECT '18015376320243459' = 18015376320243459;
        -> 0

之所以会发生这种情况是因为转换成 floating-point numbers 进行比较，只有 53 bits of precision （精度）

mysql> SELECT '18015376320243459'+0.0;
        -> 1.8015376320243e+16

显式转换解决这个问题

mysql> SELECT CAST('18015376320243459' AS UNSIGNED) = 18015376320243459;
        -> 1

通过括号改变优先级

mysql> SELECT 1+2*3;
        -> 7
mysql> SELECT (1+2)*3;
        -> 9

比较的结果是 1 (TRUE), 0 (FALSE), or NULL

默认字符串比较是不区分大小写

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 安全比较，和 = 类似，当都为 NULL 时返回1，任意一个为 NULL 时返回0

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

<>, !=

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

<=,<,>=,>

表达式	等价
(a, b) = (x, y)	(a = x) AND (b = y)
(a, b) <=> (x, y)	(a <=> x) AND (b <=> y)
(a, b) <> (x, y) and (a, b) != (x, y)	(a <> x) OR (b <> y)
(a, b) <= (x, y)	(a < x) OR ((a = x) AND (b <= y))
(a, b) < (x, y)	(a < x) OR ((a = x) AND (b < y))
(a, b) >= (x, y)	(a > x) OR ((a = x) AND (b >= y))
(a, b) > (x, y)	(a > x) OR ((a = x) AND (b > y))
expr BETWEEN min AND max	(min <= expr AND expr <= max)
expr NOT BETWEEN min AND max	NOT (expr BETWEEN min AND max)

IS and IS NOT and IS NULL and IS NOT NULL

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
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

COALESCE(value,…) 返回第一个不为 NULL 的值

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

LEAST(value1,value2,…) 返最小值
GREATEST(value1,value2,…) 返最大值，返回 NULL 若其中一个为 NULL

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'

expr IN (value,…) expr NOT IN (value,…) 存在返回1，不存在返回0

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

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

不要混合带引号和不带引号

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');

ISNULL(expr)

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

INTERVAL(N,N1,N2,N3,…) Returns 0 if N < N1, 1 if N < N2 and so on or -1 if N is NULL

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

NOT, !

mysql> SELECT NOT 10;
        -> 0
mysql> SELECT NOT 0;
        -> 1
mysql> SELECT NOT NULL;
        -> NULL
mysql> SELECT ! (1+1);
        -> 0
mysql> SELECT ! 1+1;
        -> 1

AND, &&

mysql> SELECT 1 AND 1;
        -> 1
mysql> SELECT 1 AND 0;
        -> 0
mysql> SELECT 1 AND NULL;
        -> NULL
mysql> SELECT 0 AND NULL;
        -> 0
mysql> SELECT NULL AND 0;
        -> 0

OR, ||

mysql> SELECT 1 OR 1;
        -> 1
mysql> SELECT 1 OR 0;
        -> 1
mysql> SELECT 0 OR 0;
        -> 0
mysql> SELECT 0 OR NULL;
        -> NULL
mysql> SELECT 1 OR NULL;
        -> 1

XOR

mysql> SELECT 1 XOR 1;
        -> 0
mysql> SELECT 1 XOR 0;
        -> 1
mysql> SELECT 1 XOR NULL;
        -> NULL
mysql> SELECT 1 XOR 1 XOR 1;
        -> 1

a XOR b equal (a AND (NOT b)) OR ((NOT a) and b)

= 在 SET 语法中赋值
:= 直接赋值

不像 = 符号，:= 符号永远不会解析成比较运算符，这就意味你可以使用它在任何合法的 SQL 语法里

mysql> SELECT @var1, @var2;
        -> NULL, NULL
mysql> SELECT @var1 := 1, @var2;
        -> 1, NULL
mysql> SELECT @var1, @var2;
        -> 1, NULL
mysql> SELECT @var1, @var2 := @var1;
        -> 1, 1
mysql> SELECT @var1, @var2;
        -> 1, 1

mysql> SELECT @var1:=COUNT(*) FROM t1;
        -> 4
mysql> SELECT @var1;
        -> 4

mysql> SELECT @var1;
        -> 4
mysql> SELECT * FROM t1;
        -> 1, 3, 5, 7

mysql> UPDATE t1 SET c1 = 2 WHERE c1 = @var1:= 1;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> SELECT @var1;
        -> 1
mysql> SELECT * FROM t1;
        -> 2, 3, 5, 7

= 符号只在两种场景中使用
* SET statement
* SET clause of an UPDATE statement

CASE value WHEN [compare_value] THEN result [WHEN [compare_value] THEN result ...] [ELSE result] END
CASE WHEN [condition] THEN result [WHEN [condition] THEN result ...] [ELSE result] END

mysql> SELECT CASE 1 WHEN 1 THEN 'one'
    ->     WHEN 2 THEN 'two' ELSE 'more' END;
        -> 'one'
mysql> SELECT CASE WHEN 1>0 THEN 'true' ELSE 'false' END;
        -> 'true'
mysql> SELECT CASE BINARY 'B'
    ->     WHEN 'a' THEN 1 WHEN 'b' THEN 2 END;
        -> NULL

IF(expr1,expr2,expr3)

mysql> SELECT IF(1>2,2,3);
        -> 3
mysql> SELECT IF(1<2,'yes','no');
        -> 'yes'
mysql> SELECT IF(STRCMP('test','test1'),'no','yes');
        -> 'no'

IFNULL(expr1,expr2)

mysql> SELECT IFNULL(1,0);
        -> 1
mysql> SELECT IFNULL(NULL,10);
        -> 10
mysql> SELECT IFNULL(1/0,10);
        -> 10
mysql> SELECT IFNULL(1/0,'yes');
        -> 'yes'

NULLIF(expr1,expr2)

mysql> SELECT NULLIF(1,1);
        -> NULL
mysql> SELECT NULLIF(1,2);
        -> 1

字符串操作函数
返回字符串长度超过限制，返回 NULL
字符位置从 1 开始
需要长度参数的函数，非整形参数将就近取整

很多函数声明略

字符串比较略

正则表达式略

函数返回值字符集略

算术运算符略

数学函数略

随机数生成 RAND(), RAND(N) N为随机数种子
若想获得随机数 R i <= R < j 用如下表达式 FLOOR(i + RAND() * (j − i))

你可以检索随机行通过这种方式

SELECT * FROM tbl_name ORDER BY RAND()
SELECT * FROM table1, table2 WHERE a=b AND c<d ORDER BY RAND() LIMIT 1000

CEIL(X) CEILING(X) 向上取整
FLOOR(X) 向下取整

ROUND(X), ROUND(X,D) 四舍五入

mysql> SELECT ROUND(-1.23);
        -> -1
mysql> SELECT ROUND(-1.58);
        -> -2
mysql> SELECT ROUND(1.58);
        -> 2
mysql> SELECT ROUND(1.298, 1);
        -> 1.3
mysql> SELECT ROUND(1.298, 0);
        -> 1
mysql> SELECT ROUND(23.298, -1);
        -> 20
mysql> SELECT ROUND(150.000,2), ROUND(150,2);
+------------------+--------------+
| ROUND(150.000,2) | ROUND(150,2) |
+------------------+--------------+
|           150.00 |          150 |
+------------------+--------------+

日期和时间函数

一次查询中多次调用获取时间函数将为同一值

SELECT NOW(), NOW();
-> '2016-12-01 16:53:16', '2016-12-01 16:53:16'

MySQL 中使用的日历略

全文搜索函数略

类型转换函数
BINARY 字符串转二进制字符串
CAST() 转换到任何指定类型
CONVERT() 转换到任何指定类型（多了字句用于不同字符集之间转换）
SELECT CONVERT(_latin1'Müller' USING utf8);

XML 函数略

位操作符和函数略

加密和压缩函数略

信息函数略
效率测试函数

mysql> SELECT BENCHMARK(1000000,ENCODE('hello','goodbye'));
+----------------------------------------------+
| BENCHMARK(1000000,ENCODE('hello','goodbye')) |
+----------------------------------------------+
|                                            0 |
+----------------------------------------------+
1 row in set (4.74 sec)

略略略

MySQL 5.7 Reference Manual Chapter 13 Functions and Operators 参考手册第十三章函数与操作符内容总结

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