Novice tutorial:
MySQL functions
IN 、NOT IN 、 EXISTS 、 NOT EXISTS
Sub-related query data
Practical application: filter out the data in the specified range
EXISTS and NOT EXISTS of mysql sub-associative query (1)
EXISTS and NOT EXISTS of mysql sub-associative query (two)
CAST (expression AS data_type)
Convert data type
Practical application: the field to be sorted is varchar, and the data type needs to be converted
Time conversion, range search, can also be processed by the DATE_FORMAT(d,f) function
The CAST function is used to explicitly convert an expression of a certain data type to another data type. The parameter of the CAST() function is an expression that includes the source value and target data type separated by the AS keyword.
expression: Any valid SQServer expression.
AS: Used to separate two parameters, before AS is the data to be processed, and after AS is the data type to be converted.
data_type: The data types provided by the target system, including bigint and sql_variant, cannot use user-defined data types.The types that can be converted are limited. This type can be one of the following values:
- Binary, with the effect of binary prefix: BINARY
- Character type, with parameters: CHAR()
- Date: DATE
- Time: TIME
- Date time type: DATETIME
- Floating point number: DECIMAL
- Integer: SIGNED
- Unsigned integer: UNSIGNED
The difference between cast and convert
example:
#Get current dateSELECT CAST(NOW() AS DATE)；
->2019-02-23
#Convert the value to DATETIME type
select CAST('2019-03-06 13:23:29' AS DATETIME)；
->2019-03-06 13:23:29
#Convert the value to TIME type
SELECT CAST('08:30:30' AS TIME);
->08:30:30
#varchar 转 SIGNED
SELECT CAST('6.66' AS SIGNED );
-> 6
#varchar to DECIMAL
SELECT CAST('9.12345' AS DECIMAL );
->9
#varchar to DECIMAL
# Precision and decimal places are 5 and 2 respectively; precision is the total number of digits, including the sum of the digits to the left and right of the decimal point. And the number of decimal places is the number of digits to the right of the decimal point
SELECT CAST('9.12345' AS DECIMAL(5,2) );
->9.12
The difference between CAST and DATE_FORMAT time handling
CAST is more convenient, (available DATETIME OR DATE)
CAST :
SELECT login_code, last_login_date FROM user WHERE last_login_date <= CAST('2019-02-01 14:38:39' AS DATETIME);DATE_FORMAT :
SELECT login_code, last_login_date FROM user WHERE last_login_date <= DATE_FORMAT('2019-02-01 04:38:39','%y-%m-%d %H:%i:%S');Same result:
login_code last_login_date YT_admin 2019-01-31 14:38:39 lampoy02 2019-01-31 11:42:47
BETWEEN value1 AND value2
SQL BETWEEN Condition will return the records whose expression is in the range of value1 and value2 (inclusive)
实际应用：数值区间、日期区间范围搜索
对于SQL Server，PostgreSQL和SQLite：
SELECT * FROM orders WHERE order_date BETWEEN '2016/04/19' AND '2016/05/01';对于Oracle（使用TO_DATE函数）：
SELECT * FROM orders WHERE order_date BETWEEN TO_DATE ('2016/04/19', 'yyyy/mm/dd') AND TO_DATE ('2016/05/01', 'yyyy/mm/dd');对于MySQL和MariaDB（使用CAST函数）：
SELECT * FROM orders WHERE order_date BETWEEN CAST('2016/04/19' AS DATE) AND CAST('2016/05/01' AS DATE);
IF(expr,v1,v2)
如果表达式 expr 成立，返回结果 v1；否则，返回结果 v2。
实际应用：树的子节点判断，true or false
SELECT IF(1 > 0,'正确','错误') ->正确
IFNULL(v1,v2)
如果 v1 的值为 NULL，则返回 v2，否则返回 v1。
SELECT IFNULL(null,'Hello Word') ->Hello Word SELECT IFNULL(1,'Hello Word') ->1 SELECT IFNULL(1/0,'Hello Word') ->Hello Word
ISNULL(expression)
判断表达式是否为 NULL
实际应用：可与 IF(expr,v1,v2) 函数结合使用
SELECT ISNULL(NULL); ->1 SELECT ISNULL(' '); ->0
CASE
CASE 表示函数开始，END 表示函数结束。如果 condition1 成立，则返回 result1, 如果 condition2 成立，则返回 result2，当全部不成立则返回 result，而当有一个成立之后，后面的就不执行了。
MySQL 的CASE WHEN 语句使用说明
Mysql查询时case when语句的使用
第一种用法：case后面跟列名，when后面跟对应值
CASE case_value WHEN when_value THEN statement_list [WHEN when_value THEN statement_list] ... [ELSE statement_list] END这种用法正好解决我们刚刚提出的问题，当sex值为0时当前列显示“男”，否则显示“女”，sql写法如下：
SELECT ( CASE sex WHEN 0 THEN '男' ELSE '女' END ) AS 性别, avg(score) AS 平均分 FROM grade GROUP BY sex; +--------+-----------+ | 性别 | 平均分 | +--------+-----------+ | 男 | 67.2500 | | 女 | 89.5000 | +--------+-----------+ 2 rows in set (0.00 sec)第二种用法：case后面空白，when后面跟着判断条件
CASE WHEN search_condition THEN statement_list [WHEN search_condition THEN statement_list] ... [ELSE statement_list] END针对于这种写法，我们考虑这样一种需求，学生成绩是有评分的，大于等于90分的学生是A，小于90分大于等于60分的学生是B， 其余的学生是C，现在要查询评分为A、B、C的学生成绩的平均分分别是多少，因为成绩评分并不是单独的一列，所以不能简单的 使用
group by来分组实现了，但是可以利用case when语句实现，写起来也很简单，看看下面的sql语句就知道了！SELECT ( CASE WHEN score >= 90 THEN 'A' WHEN score < 60 THEN 'C' ELSE 'B' END ) AS 等级, avg(score) AS 平均分 FROM grade GROUP BY 等级; +--------+-----------+ | 等级 | 平均分 | +--------+-----------+ | A | 95.0000 | | B | 74.8000 | | C | 47.3333 | +--------+-----------+ 3 rows in set (0.00 sec)
CONCAT(s1,s2...sn)
字符串 s1,s2 等多个字符串合并为一个字符
实际应用：SQL中地址的拼接（省，市，乡）
SELECT CONCAT("SQL ", "Runoob ", "Gooogle ", "Facebook") AS ConcatenatedString;
REPLACE(s,s1,s2)
将字符串 s2 替代字符串 s 中的字符串 s1
将字符串 abc 中的字符 a 替换为字符 x：
SELECT REPLACE('abc','a','x') --xbc
GROUP_CONCAT([DISTINCT] 要连接的字段 [Order BY ASC/DESC 排序字段] [Separator '分隔符'])
能将相同的行组合起来
GROUP_CONCAT([DISTINCT] expr [,expr ...] [ORDER BY {unsigned_integer | col_name | expr} [ASC | DESC] [,col_name ...]] [SEPARATOR str_val]) SELECT student_name, GROUP_CONCAT(test_score) FROM student GROUP BY student_name; SELECT student_name, GROUP_CONCAT(DISTINCT test_score ORDER BY test_score DESC SEPARATOR ' ') FROM student GROUP BY student_name;在MySQL中，您可以获得表达式组合的连接值。要消除重复值，请使用该
DISTINCT子句。要对结果中的值进行排序，请使用该ORDER BY子句。要按相反顺序排序，请将DESC（descending）关键字添加到要在ORDER BY子句中排序的列的名称中。默认为升序; 这可以使用ASC关键字明确指定。组中值之间的默认分隔符是逗号（,）。要明确指定分隔符，请使用SEPARATOR后跟应在组值之间插入的字符串文字值。要完全消除分隔符，请指定SEPARATOR ''。group_concat函数详解
GROUP_CONCAT(expr)
DATEDIFF(d1,d2)
计算日期 d1->d2 之间相隔的天数
实际应用：计算日期差值
SELECT DATEDIFF('2019-02-24','2019-02-20') -> 4
DATE_FORMAT(d,f)
按表达式 f的要求显示日期 d
实际用途：时间格式化 OR 时间区间搜索
SELECT DATE_FORMAT('2019-02-26 22:30:45','%Y-%m-%d'); ->2019-02-26 SELECT DATE_FORMAT('2019-02-26 22:30:45','%Y-%m-%d %H:%i:%S'); ->2019-02-26 22:30:45