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';-> 2mysql> 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';-> 0mysql> SELECT 7 > '6x';-> 1mysql> SELECT 0 > 'x6';-> 0mysql> 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;-> 1mysql> 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;-> 7mysql> 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;-> 0mysql> SELECT '0' = 0;-> 1mysql> SELECT '0.0' = 0;-> 1mysql> SELECT '0.01' = 0;-> 0mysql> 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, 0mysql> SELECT 1 = 1, NULL = NULL, 1 = NULL;-> 1, NULL, NULL
<>, !=
mysql> SELECT '.01' <> '0.01';-> 1mysql> SELECT .01 <> '0.01';-> 0mysql> SELECT 'zapp' <> 'zappp';-> 1
<=,<,>=,>
IS and IS NOT and IS NULL and IS NOT NULL
mysql> SELECT 1 IS TRUE, 0 IS FALSE, NULL IS UNKNOWN;-> 1, 1, 1mysql> SELECT 1 IS NOT UNKNOWN, 0 IS NOT UNKNOWN, NULL IS NOT UNKNOWN;-> 1, 1, 0mysql> SELECT 1 IS NULL, 0 IS NULL, NULL IS NULL;-> 0, 0, 1mysql> 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);-> 1mysql> 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);-> 2mysql> SELECT GREATEST(34.0,3.0,5.0,767.0);-> 767.0mysql> 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);-> 0mysql> SELECT 'wefwf' IN ('wee','wefwf','weg');-> 1mysql> SELECT (3,4) IN ((1,2), (3,4));-> 1mysql> 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);-> 0mysql> 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);-> 3mysql> SELECT INTERVAL(10, 1, 10, 100, 1000);-> 2mysql> SELECT INTERVAL(22, 23, 30, 44, 200);-> 0
NOT, !
mysql> SELECT NOT 10;-> 0mysql> SELECT NOT 0;-> 1mysql> SELECT NOT NULL;-> NULLmysql> SELECT ! (1+1);-> 0mysql> SELECT ! 1+1;-> 1
AND, &&
mysql> SELECT 1 AND 1;-> 1mysql> SELECT 1 AND 0;-> 0mysql> SELECT 1 AND NULL;-> NULLmysql> SELECT 0 AND NULL;-> 0mysql> SELECT NULL AND 0;-> 0
OR, ||
mysql> SELECT 1 OR 1;-> 1mysql> SELECT 1 OR 0;-> 1mysql> SELECT 0 OR 0;-> 0mysql> SELECT 0 OR NULL;-> NULLmysql> SELECT 1 OR NULL;-> 1
XOR
mysql> SELECT 1 XOR 1;-> 0mysql> SELECT 1 XOR 0;-> 1mysql> SELECT 1 XOR NULL;-> NULLmysql> 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, NULLmysql> SELECT @var1 := 1, @var2;-> 1, NULLmysql> SELECT @var1, @var2;-> 1, NULLmysql> SELECT @var1, @var2 := @var1;-> 1, 1mysql> SELECT @var1, @var2;-> 1, 1mysql> SELECT @var1:=COUNT(*) FROM t1;-> 4mysql> SELECT @var1;-> 4mysql> SELECT @var1;-> 4mysql> SELECT * FROM t1;-> 1, 3, 5, 7mysql> UPDATE t1 SET c1 = 2 WHERE c1 = @var1:= 1;Query OK, 1 row affected (0.00 sec)Rows matched: 1 Changed: 1 Warnings: 0mysql> SELECT @var1;-> 1mysql> SELECT * FROM t1;-> 2, 3, 5, 7
= 符号只在两种场景中使用
SET statementSET clause of an UPDATE statement
CASE value WHEN [compare_value] THEN result [WHEN [compare_value] THEN result ...] [ELSE result] ENDCASE 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);-> 3mysql> SELECT IF(1<2,'yes','no');-> 'yes'mysql> SELECT IF(STRCMP('test','test1'),'no','yes');-> 'no'
IFNULL(expr1,expr2)
mysql> SELECT IFNULL(1,0);-> 1mysql> SELECT IFNULL(NULL,10);-> 10mysql> SELECT IFNULL(1/0,10);-> 10mysql> SELECT IFNULL(1/0,'yes');-> 'yes'
NULLIF(expr1,expr2)
mysql> SELECT NULLIF(1,1);-> NULLmysql> 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 Ri <= 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);-> -1mysql> SELECT ROUND(-1.58);-> -2mysql> SELECT ROUND(1.58);-> 2mysql> SELECT ROUND(1.298, 1);-> 1.3mysql> SELECT ROUND(1.298, 0);-> 1mysql> SELECT ROUND(23.298, -1);-> 20mysql> 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();-> '-12-01 16:53:16', '-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