How to perform type conversion in mysql

Conversion method: 1. Use the " " operator, the syntax is "SELECT 1 'String';"; 2. Use the CAST() function to convert any type to a specified type, the syntax is "CAST(expr AS type)"; 3. Use the DATE_FORMAT() function to convert the date into a string according to the given pattern.

The operating environment of this tutorial: windows7 system, mysql8 version, Dell G3 computer.

1. The concept of implicit type conversion and explicit type conversion

Implicit type conversion:

To perform operations or comparisons between two values, the data types must first be consistent. Implicit type conversion occurs when two data types are found to be inconsistent. For example, convert a string into a number, or vice versa:

SELECT 1+'1'; – 字符串1转成数字
SELECT concat(2,'test'); – 数字2转成字符串

Explicit type conversion:

Use functions to convert data types

2.Cast function

##CAST(expr AS type)

Convert any type of expression expr into a value of the specified type type. type can be any of the following types:

• BINARY[(N)]: binary string, the length after conversion is less than N bytes (if the length is less than N, 0x00 will be added at the end)

• CHAR[(N)]: String, the length after conversion is less than N characters

• DATE: Date

• DATETIME: Date and time

• DECIMAL[(M[,N])]: Floating point number, M is the total number of digits (including the integer part and the decimal part) , N is the number of digits after the decimal point

##SIGNED [INTEGER]: Signed integer
TIME: Time
UNSIGNED [INTEGER]: Unsigned integer

Convert string to number

When converting to decimal, scan characters from the beginning String up to the first non-numeric character. Round the truncated digit

1) By default, M and N are not limited, converted to an integer, and rounded to the first decimal place:

SELECT cast(‘28.82abc’ AS DECIMAL);

2) Limit M but not limit N, convert it to an integer (M should be as large as possible):

SELECT cast(‘28.82abc’ AS DECIMAL(4));

If M When the length is less than the actual number of digits, it will be converted to the maximum value of the set number of digits. In the following conversion, M is 1, that is, the two-digit number 29 cannot be displayed, and the largest one-digit number, 9

SELECT cast(‘28.82abc’ AS DECIMAL(1));

, is limited to M and N, which must be First satisfy N decimal places after the decimal point. In the following example, 1 decimal place is satisfied first, and then 1 integer is taken:

SELECT cast(‘28.82abc’ AS DECIMAL(2,1));

2 decimal places are satisfied first, and the integer part is not taken:

SELECT cast(‘28.82abc’ AS DECIMAL(2,2));

SELECT cast(‘28.82abc’ AS UNSIGNED);

SELECT cast(’-28abc’ AS SIGNED);

##String to date

SELECT cast(‘2007-12-25’ AS DATETIME);

SELECT cast(‘2007-12-25’ AS DATE);

SELECT cast(‘20:20:20’ AS TIME);

The date format must be 'YYYY-MM-DD'

SELECT cast(‘25-12-2007’ AS DATETIME);
SELECT cast(‘2007-may-25’ AS DATETIME);

Number to string

SELECT cast(123 AS CHAR);
 SELECT cast(123 AS BINARY);

Date to string

SELECT cast(now() AS CHAR);

String to binary string

二进制字符串：二进制字符串被视为一个连续的字节序列，与字符集无关。非二进制字符串（即我们通常所说的字符串）被视为一个连续排列的字符序列，与字符集有关。所谓与字符集无关，是指与MySQL自己的字符集无关，而是按照操作系统的字符集把字符串转换成字节进行存储

两种字符串的比较方式：二进制字符串的比较方式是一个字节一个字节进行的，比较的依据是两个字节的二进制值。因为同一个字母的大小写的数值编码是不一样的，因此它是区分大小写的。另外，由于它和字符集无关，因此也就没有大写和小写字母一说

非二进制字符串的比较方式是一个字符一个字符进行的，比较的依据是两个字符在字符集中的先后顺序。根据使用的校对不同，可以进行区分大小写的比较和不区分大小写的比较

使用CAST(str AS BINARY)将字符串转换成二进制字符串时，通常使用它的快捷方式写法： BINARY str

查看当前字符集和校对规则设置

SHOW VARIABLES LIKE ‘collation_%’;

在比较表达式中，binary影响后面所有的字符串，并且不会忽略字符串的尾部空格

SELECT ‘a’ = ‘A’ ,BINARY ‘a’ = ‘A’;
 SELECT ‘a’ = 'a ',BINARY ‘a’ = 'a ';

Cast(col_name as date_type)

将指定的列或者表达式转换为指定的数据类型

使用场景:当两个值进行比较,但是数据源的表中列的数据不一致,这个时候可以使用cast函数进行转化

3.Convert函数

CONVERT(expr, type), CONVERT(expr USING sharset_name)

convert函数的作用和cast函数几乎相同，但是它可以把字符串从一种字符集转换成另一种字符集。下例将字符串’abc’从默认的字符集转换成utf8字符集

SELECT convert(‘abc’ USING utf8);

SELECT convert(‘你好’ USING gbk);

如果目标字符集不能表示该字符，则返回乱码

SELECT convert(‘你好’ USING latin1);

日期字符串转换函数

DATE_FORMAT(date, format)

将日期date按照给定的模式format转换成字符串。format中可使用以下模式元素

SELECT date_format(‘2009-10-04 22:23:00’, ‘%W %M %Y’);

SELECT date_format(now(), ‘%W %M %Y’);

SELECT date_format(‘2007-10-04 22:23:00’, ‘%H:%i:%s’);

TIME_FORMAT(date, format)

format中只可使用时、分、秒和微秒模式元素

SELECT time_format(‘22:23:01’, ‘%H.%i.%s’);

STR_TO_DATE(str, format)

将字符串str以指定的模式format转换成日期。format中可以包含模式元素和字面量，字面量必须匹配str中的字面量：

SELECT str_to_date(‘01,5,2013’,’%d,%m,%Y’);
SELECT str_to_date(‘May 1, 2013’,’%M %d,%Y’);

以下在会话变量@@sql_mode设置中包含no_zero_date和no_zero_in_date时转换失败，没有包含这些设置时转换成功

SELECT str_to_date(‘a09:30:17’,‘a%h:%i:%s’);
 SELECT str_to_date(‘09:30:17a’,’%h:%i:%s’);

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