MySQL 中的 base64 函數

MySQL在5.6版本的時候增加了to_base64和from_base64函數，在此之前沒有內部函數，只能使用User-Defined-Function。
本文分別介紹這兩種方式下base64函數的使用。

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1. 5.6版本及之後的版本的base64

主要就是兩個MySQL內部函數to_base64和from_base64，使用也很簡單，如下：

mysql> select version();
+-----------+
| version() |
+-----------+
| 5.7.11    |
+-----------+
1 row in set (0.00 sec)

mysql> select to_base64('helloworld');
+-------------------------+
| to_base64('helloworld') |
+-------------------------+
| aGVsbG93b3JsZA==        |
+-------------------------+
1 row in set (0.00 sec)

mysql> select from_base64('aGVsbG93b3JsZA==');
+---------------------------------+
| from_base64('aGVsbG93b3JsZA==') |
+---------------------------------+
| helloworld                      |
+---------------------------------+
1 row in set (0.00 sec)

mysql> 

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2. 5.6之前的版本

由於之前版本MySQL沒有與base64相關的函數，就有了很多用戶自己實現了這兩個函數base64_encode、base64_decode，下面是一個比較好的實現(http://www.cybercanibal.com/articulos-tecnicos/experiencia-tecnica/115-mysql-url-base64-encode-decode-function)：

-- base64.sql - MySQL base64 encoding/decoding functions
-- Copyright (C) 2006 Ian Gulliver
-- 
-- This program is free software; you can redistribute it and/or modify
-- it under the terms of version 2 of the GNU General Public License as
-- published by the Free Software Foundation.
-- 
-- This program is distributed in the hope that it will be useful,
-- but WITHOUT ANY WARRANTY; without even the implied warranty of
-- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-- GNU General Public License for more details.
-- 
-- You should have received a copy of the GNU General Public License
-- along with this program; if not, write to the Free Software
-- Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA

delimiter |

DROP TABLE IF EXISTS input_pentaho_bod.base64_data |
CREATE TABLE base64_data (c CHAR(1) BINARY, val TINYINT) |
INSERT INTO base64_data VALUES 
    ('A',0), ('B',1), ('C',2), ('D',3), ('E',4), ('F',5), ('G',6), ('H',7), ('I',8), ('J',9),
    ('K',10), ('L',11), ('M',12), ('N',13), ('O',14), ('P',15), ('Q',16), ('R',17), ('S',18), ('T',19),
    ('U',20), ('V',21), ('W',22), ('X',23), ('Y',24), ('Z',25), ('a',26), ('b',27), ('c',28), ('d',29),
    ('e',30), ('f',31), ('g',32), ('h',33), ('i',34), ('j',35), ('k',36), ('l',37), ('m',38), ('n',39),
    ('o',40), ('p',41), ('q',42), ('r',43), ('s',44), ('t',45), ('u',46), ('v',47), ('w',48), ('x',49),
    ('y',50), ('z',51), ('0',52), ('1',53), ('2',54), ('3',55), ('4',56), ('5',57), ('6',58), ('7',59),
    ('8',60), ('9',61), ('+',62), ('/',63), ('=',0) |


DROP FUNCTION IF EXISTS input_pentaho_bod.BASE64_DECODE |
CREATE FUNCTION BASE64_DECODE (input BLOB)
    RETURNS BLOB
    CONTAINS SQL
    DETERMINISTIC
    SQL SECURITY INVOKER
BEGIN
    DECLARE ret BLOB DEFAULT '';
    DECLARE done TINYINT DEFAULT 0;

    IF input IS NULL THEN
        RETURN NULL;
    END IF;

each_block:
    WHILE NOT done DO BEGIN
        DECLARE accum_value BIGINT UNSIGNED DEFAULT 0;
        DECLARE in_count TINYINT DEFAULT 0;
        DECLARE out_count TINYINT DEFAULT 3;

each_input_char:
        WHILE in_count < 4 DO BEGIN
            DECLARE first_char CHAR(1);

            IF LENGTH(input) = 0 THEN
                RETURN ret;
            END IF;

            SET first_char = SUBSTRING(input,1,1);
            SET input = SUBSTRING(input,2);

            BEGIN
                DECLARE tempval TINYINT UNSIGNED;
                DECLARE error TINYINT DEFAULT 0;
                DECLARE base64_getval CURSOR FOR SELECT val FROM base64_data WHERE c = first_char;
                DECLARE CONTINUE HANDLER FOR SQLSTATE '02000' SET error = 1;

                OPEN base64_getval;
                FETCH base64_getval INTO tempval;
                CLOSE base64_getval;

                IF error THEN
                    ITERATE each_input_char;
                END IF;

                SET accum_value = (accum_value << 6) + tempval;
            END;

            SET in_count = in_count + 1;

            IF first_char = '=' THEN
                SET done = 1;
                SET out_count = out_count - 1;
            END IF;
        END; END WHILE;

        -- We've now accumulated 24 bits; deaccumulate into bytes

        -- We have to work from the left, so use the third byte position and shift left
        WHILE out_count > 0 DO BEGIN
            SET ret = CONCAT(ret,CHAR((accum_value & 0xff0000) >> 16));
            SET out_count = out_count - 1;
            SET accum_value = (accum_value << 8) & 0xffffff;
        END; END WHILE;

    END; END WHILE;

    RETURN ret;
END |

DROP FUNCTION IF EXISTS input_pentaho_bod.BASE64_ENCODE |
CREATE FUNCTION BASE64_ENCODE (input BLOB)
    RETURNS BLOB
    CONTAINS SQL
    DETERMINISTIC
    SQL SECURITY INVOKER
BEGIN
    DECLARE ret BLOB DEFAULT '';
    DECLARE done TINYINT DEFAULT 0;

    IF input IS NULL THEN
        RETURN NULL;
    END IF;

each_block:
    WHILE NOT done DO BEGIN
        DECLARE accum_value BIGINT UNSIGNED DEFAULT 0;
        DECLARE in_count TINYINT DEFAULT 0;
        DECLARE out_count TINYINT;

each_input_char:
        WHILE in_count < 3 DO BEGIN
            DECLARE first_char CHAR(1);

            IF LENGTH(input) = 0 THEN
                SET done = 1;
                SET accum_value = accum_value << (8 * (3 - in_count));
                LEAVE each_input_char;
            END IF;

            SET first_char = SUBSTRING(input,1,1);
            SET input = SUBSTRING(input,2);

            SET accum_value = (accum_value << 8) + ASCII(first_char);

            SET in_count = in_count + 1;
        END; END WHILE;

        -- We've now accumulated 24 bits; deaccumulate into base64 characters

        -- We have to work from the left, so use the third byte position and shift left
        CASE
            WHEN in_count = 3 THEN SET out_count = 4;
            WHEN in_count = 2 THEN SET out_count = 3;
            WHEN in_count = 1 THEN SET out_count = 2;
            ELSE RETURN ret;
        END CASE;

        WHILE out_count > 0 DO BEGIN
            BEGIN
                DECLARE out_char CHAR(1);
                DECLARE base64_getval CURSOR FOR SELECT c FROM base64_data WHERE val = (accum_value >> 18);

                OPEN base64_getval;
                FETCH base64_getval INTO out_char;
                CLOSE base64_getval;

                SET ret = CONCAT(ret,out_char);
                SET out_count = out_count - 1;
                SET accum_value = accum_value << 6 & 0xffffff;
            END;
        END; END WHILE;

        CASE
            WHEN in_count = 2 THEN SET ret = CONCAT(ret,'=');
            WHEN in_count = 1 THEN SET ret = CONCAT(ret,'==');
            ELSE BEGIN END;
        END CASE;

    END; END WHILE;

    RETURN ret;
END |
delimiter ;

使用也非常簡單：

mysql> select version();
+------------+
| version()  |
+------------+
| 5.5.45-log |
+------------+
1 row in set (0.00 sec)

mysql> select base64_encode('helloworld');
+-----------------------------+
| base64_encode('helloworld') |
+-----------------------------+
| aGVsbG93b3JsZA==            |
+-----------------------------+
1 row in set (0.00 sec)

mysql> select base64_decode('aGVsbG93b3JsZA==');
+-----------------------------------+
| base64_decode('aGVsbG93b3JsZA==') |
+-----------------------------------+
| helloworld                        |
+-----------------------------------+
1 row in set (0.00 sec)

mysql> 

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3. 性能評估

UDF的性能比native function的性能差上90-100倍，其實還有一種方式[C版本的的UDF]（http://bugs.mysql.com/file.php?id=3294&bug_id=18861），C版本的的UDF在性能上沒有問題，但是也會存在部署和移植的問題，所幸的是在新版本的MySQL（5.6+）中已經有了這兩個函數to_base64和from_base64。

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ref:

http://stackoverflow.com/questions/358500/base64-encode-in-mysql
http://lists.mysql.com/internals/37665
http://www.cybercanibal.com/articulos-tecnicos/experiencia-tecnica/115-mysql-url-base64-encode-decode-function
http://dev.mysql.com/doc/refman/5.6/en/string-functions.html#function_to-base64
http://dev.mysql.com/doc/refman/5.6/en/string-functions.html#function_from-base64