MySQL Basics Tutorial 11 - Cast Functions and Operators of Functions

• BINARY

The BINARY operator throws the following string into a binary string. This is a simple way to force column comparisons to be done byte-by-byte rather than character-by-character. This makes the comparison case-sensitive, even if the column is not defined as a BINARY or BLOB. BINARY also produces trailing whitespace, making it more conspicuous.

mysql> SELECT 'a' = 'A';
-> 1
mysql> SELECT BINARY 'a' = 'A';
-> 0
mysql> SELECT 'a' = 'a ';
-> 1
mysql> SELECT BINARY 'a' = 'a ';
-> 0

BINARY affects the entire comparison; it can be given before any operand and produce the same result.

BINARY str is the abbreviation of CAST (str AS BINARY).

Note that in some contexts, if you assign an indexed column to BINARY, MySQL will not be able to effectively use the index.

If you want to perform a case-sensitive comparison of a BLOB value or other binary string, you can do this by taking advantage of the fact that binary strings do not have character sets, so there are no folders. concept. To perform a case-sensitive comparison, use the CONVERT() function to convert a string value to a case-insensitive character set. The result is a non-binary string, so the LIKE operation is not case-sensitive:

SELECT 'A' LIKE CONVERT(blob_col USING latin1) FROM tbl_name ;

To use a different character set, replace its latin1 name in the above statement.

CONVERT() can generally be used to compare strings that appear in different character sets.

• CAST(expr AS type), CONVERT(expr,type) , CONVERT(expr USING transcoding_name)

type can be one of the following values:

##BINARY[(
• N

  )]

CHAR[(
• N

  )]

  ##DATE

• DATETIME

• DECIMAL

• SIGNED [INTEGER]

• TIME

• UNSIGNED [INTEGER]
BINARY Generates a binary string. See the BINARY operator entry in this chapter for a description of how it affects the comparison result.

N

N] causes the cast to use this parameter for no more than N bytes . Likewise, CHAR[N] will cause the cast to use no more than N characters. CAST() and CONVERT(... USING...) are standard SQL syntax. The non-USING format of CONVERT() is ofis ODBC syntax.

enum_col

tbl_name ORDER BY CAST(enum_col AS CHAR);CAST(str

str. CAST(expr AS CHAR) Treats the expression as a string with the default character set. If used as part of a more complex expression such as CONCAT('Date: ',CAST(NOW() AS DATE)), CAST() will also change the result.

mysql> SELECT 1+'1';
-> 2

mysql> SELECT CONCAT('hello you ',2);
-> 'hello you 2'

mysql> SELECT CAST(1-2 AS UNSIGNED)
-> 18446744073709551615
mysql> SELECT CAST(CAST(1-2 AS UNSIGNED) AS SIGNED);
-> -1

mysql> SELECT CAST(1 AS UNSIGNED) - 2.0;
-> -1.0

若你在一个算术运算中使用了一个字符串，它会被转化为一个浮点数。

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