从字符串 MySQL 中选择前 4 个数字

Question

如果我有这样的字符串：

CC123484556
CC492014512
BUXT122256690

如何在 MySQL 中操作这样的代码来提取前 4 个数字值？其他行中的数字前有各种 # 个字母，但最重要的是出现的前 4 个数字。

SELECT LEFT(alloy , 4) FROM tbl 

所以想要的结果是：

1234
4920
1222

Answer 1

又慢又丑：

SELECT col,
       SUBSTRING(tab.col, MIN(LOCATE(four_digits, tab.col,1)), 4) + 0 AS result
FROM  (SELECT 'CC123484556' AS col UNION ALL
       SELECT 'CC492014512' UNION ALL
       SELECT 'BUXT122256690' UNION ALL
       SELECT 'abced') tab
CROSS JOIN (
   SELECT  CONCAT(d1.z, d2.z, d3.z, d4.z) AS four_digits
   FROM (SELECT '1' AS z UNION SELECT '2' UNION SELECT '3' UNION
            SELECT '4' UNION SELECT '5' UNION SELECT '6' UNION
            SELECT '7' UNION SELECT '8' UNION SELECT '9' UNION SELECT '0') d1
  CROSS JOIN (SELECT '1' AS z UNION SELECT '2' UNION SELECT '3' UNION
            SELECT '4' UNION SELECT '5' UNION SELECT '6' UNION
            SELECT '7' UNION SELECT '8' UNION SELECT '9' UNION SELECT '0') d2
  CROSS JOIN (SELECT '1' AS z UNION SELECT '2' UNION SELECT '3' UNION
            SELECT '4' UNION SELECT '5' UNION SELECT '6' UNION
            SELECT '7' UNION SELECT '8' UNION SELECT '9' UNION SELECT '0') d3
  CROSS JOIN (SELECT '1' AS z UNION SELECT '2' UNION SELECT '3' UNION
            SELECT '4' UNION SELECT '5' UNION SELECT '6' UNION
            SELECT '7' UNION SELECT '8' UNION SELECT '9' UNION SELECT '0') d4
 ) sub
WHERE LOCATE(four_digits, tab.col,1) > 0
GROUP BY col;

Rextester Demo

生成所有 4 位数字组合，在字符串中定位它们并获取索引最低的子字符串。

编辑：

更快一点的方法：

SELECT col, SUBSTRING(col, MIN(i), 4) + 0 AS r
FROM (
    SELECT col, SUBSTRING(tab.col, i , 4) + 0 AS result, i
    FROM tab
    CROSS JOIN (
       SELECT  CONCAT(d1.z, d2.z)+1 AS i
       FROM (SELECT '1' AS z UNION SELECT '2' UNION SELECT '3' UNION
                SELECT '4' UNION SELECT '5' UNION SELECT '6' UNION
                SELECT '7' UNION SELECT '8' UNION SELECT '9' UNION SELECT '0') d1
      CROSS JOIN (SELECT '1' AS z UNION SELECT '2' UNION SELECT '3' UNION
                SELECT '4' UNION SELECT '5' UNION SELECT '6' UNION
                SELECT '7' UNION SELECT '8' UNION SELECT '9' UNION SELECT '0') d2
     ) sub
     WHERE  i <= LENGTH(tab.col)-1
) sub
WHERE result <> 0
GROUP BY col;

Rextester Demo2

从头开始获取4个字符的子串，隐式转换为数字，获取i最低的数字。

Answer 2

使用locate()、least() 和substr()

SQLFiddle Demo

select col,SUBSTR(col,LEAST(
    if (Locate(0,col) >0,Locate(0,col),999),
    if (Locate(1,col) >0,Locate(1,col),999),
    if (Locate(2,col) >0,Locate(2,col),999),
    if (Locate(3,col) >0,Locate(3,col),999),
    if (Locate(4,col) >0,Locate(4,col),999),
    if (Locate(5,col) >0,Locate(5,col),999),
    if (Locate(6,col) >0,Locate(6,col),999),
    if (Locate(7,col) >0,Locate(7,col),999),
    if (Locate(8,col) >0,Locate(8,col),999),
    if (Locate(9,col) >0,Locate(9,col),999)
  ),4) as result from test;

测试结果：

mysql> create table test ( col varchar(15));
Query OK, 0 rows affected (0.70 sec)

mysql> insert into test (col) values 
    -> ('CC123484556'),
    -> ('CC492014512'),
    -> ('BUXT122256690');
Query OK, 3 rows affected (0.13 sec)
Records: 3  Duplicates: 0  Warnings: 0

输出：

mysql> select * from test;
+---------------+
| col           |
+---------------+
| CC123484556   |
| CC492014512   |
| BUXT122256690 |
+---------------+
3 rows in set (0.00 sec)

mysql> select col,SUBSTR(col,LEAST(
    ->     if (Locate(0,col) >0,Locate(0,col),999),
    ->     if (Locate(1,col) >0,Locate(1,col),999),
    ->     if (Locate(2,col) >0,Locate(2,col),999),
    ->     if (Locate(3,col) >0,Locate(3,col),999),
    ->     if (Locate(4,col) >0,Locate(4,col),999),
    ->     if (Locate(5,col) >0,Locate(5,col),999),
    ->     if (Locate(6,col) >0,Locate(6,col),999),
    ->     if (Locate(7,col) >0,Locate(7,col),999),
    ->     if (Locate(8,col) >0,Locate(8,col),999),
    ->     if (Locate(9,col) >0,Locate(9,col),999)
    ->   ),4) as result from test;
+---------------+--------+
| col           | result |
+---------------+--------+
| CC123484556   | 1234   |
| CC492014512   | 4920   |
| BUXT122256690 | 1222   |
+---------------+--------+
3 rows in set (0.00 sec)