MySQL -- implicit conversion of data type

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When two different types of data are operated on , In order to make them compatible ,MySQL Implicit data type conversions may be performed （Implicit type conversion）

One 、MySQL Automatically convert strings to numbers

• The string starts with a number , And all are numbers , Then the numerical result of conversion is the whole string
• The string starts with a number , And some are numbers , Then the converted numerical result is as of the first character that is not a number
• The string does not start with a number , Then the numerical result of conversion is 0

--  The string starts with a number , And all are numbers 
SET @str1 := '123';
SELECT @str1 = 123;--  result ：1, representative true
SELECT @str1 = '123';--  result ：1 representative true
SELECT @str1 + 1;--  result ：124

--  The string starts with a number , And some are numbers 
SET @str2 := '123abc';
SELECT @str2 = 123;--  result ：1, representative true
SELECT @str2 = '123';--  result ：0, representative false
SELECT @str2 + 1;--  result ：124

--  The string does not start with a number 
SET @str3 := 'abc123';
SELECT @str3 = 123;--  result ：0, representative false
SELECT @str3 = '123';--  result ：0, representative false
SELECT @str3 + 100;--  result ：100

Two 、MySQL Use functions to convert numbers to characters

-- CAST() Function to explicitly convert a number to a string 
SELECT 1.1, CAST(1.1 AS CHAR);

-- CONCAT() Function implicitly converts a number to a string 
SELECT 1.1, CONCAT(1.1);

3、 ... and 、 The official document describes implicit conversion

【 official 】type-conversion

The following rules describe how the comparison operation is transformed ：

• At least one of the two parameters is null when , The result of the comparison is null, In addition, use <=> The two one. null When making a comparison, it will return 1, There is no need for type conversion in either case
• Both parameters are strings , Compare by string , No type conversion
• Both arguments are integers , Compare... By integers , No type conversion
• When comparing hexadecimal values with non numeric values , It will be treated as a binary string
• One parameter is timestamp perhaps datetime, And the other parameter is constant , Constants are converted to timestamp
• One parameter is decimal type , If the other parameter is decimal Or an integer , Will convert integers to decimal Then compare , If the other parameter is a floating-point number , It will put decimal Convert to floating point for comparison
• In other cases , Both parameters are converted to floating-point numbers for comparison