string - Oracle:获取部分字符串匹配的长度

Question

想象一下我有一张像

Name
----
ABCDEFG
ABChello world
ABCDEfoo
ABbar
ABCDEF
ABCDEFGHIJKLMNOP
zzz
qABCD
ABCqqqGH
ABCABC

我想做一个查询并找出每个字符串有多少个字符与所需的字符串“ABCDEFGHIJ”匹配，总是从头开始。那是...

Name               MatchingLength
----               ----
ABCDEFG            7
ABChello world     3
ABCDEzoo           5
ABbar              2
ABCDEF             6
ABCDEFGHIJKLMNOP   10
zzz                0
qABCD              0
ABCqqqGH           3
ABCABC             3

有什么办法可以在 Oracle 中干净利落地做到这一点？我不知所措。

Answer 1

不知道“干净”，但这里有两个解决方案。

-- The hardcoded, bad performance. No transformation of your string though.
with patterns as (
        select substr('ABCDEFGHIJ', 1, rownum) txt
        from dual 
        connect by level <= length('ABCDEFGHIJ')
    )
select d.txt, coalesce(max(length(p.txt)), 0)
from dummy d
    left join patterns p
        on instr(d.txt, p.txt) = 1
group by d.txt
order by 2 desc;

-- The cool one with regex. 
-- Though transforming your input string, 
-- this can also be done with ease making something that transorms it for you
-- like in the previous example, more complicated task than the previous,
-- as oracle sucks with string manipulation. You can however write it in java.
select d.txt, coalesce(LENGTH(REGEXP_SUBSTR(d.txt, '^A(B(C(D(E(F(G(H(I(J)?)?)?)?)?)?)?)?)')), 0)
from dummy d;

http://www.sqlfiddle.com/#!4/85ba6/23

更新

with patterns as (
        select substr('ABCDEFGHIJ', 1, rownum) txt
        from dual 
        connect by level <= length('ABCDEFGHIJ')
    )
select d.txt, coalesce(max(length(p.txt)), 0)
from dummy d
    left join patterns p
        on instr(d.txt, p.txt) = 1
where d.txt LIKE substr('ABCDEFGHIJ', 1, 1) || '%'
group by d.txt
order by 2 desc;

更新 fiddle :http://www.sqlfiddle.com/#!4/37400/6

在 oracle 10g 上测试生成的查询计划

SELECT STATEMENT, GOAL = ALL_ROWS       
 SORT ORDER BY                  
  SORT GROUP BY NOSORT          
   NESTED LOOPS OUTER           
    INDEX RANGE SCAN    I <<<< Uses the index.
    VIEW                            
     COUNT                  
      CONNECT BY WITHOUT FILTERING                  
       FAST DUAL