如何将2个mysql查询与1个表合并为一个

Question

I have one table, example

catid | parentcatid | dirname
  4   |      0      | mobiles 
 243  |      4      | Nokia

When I get catid = 243 I need to take with mysql_query (catid=parentcatid)

243 | mobiles | Nokia

I can first select catid , parentcatid , and dirname where id=243 and then select the same where parentcatid = catid , but

How to make this single query ?

Answer 1

JOIN the table twice:

SELECT
  t2.catid, t1.dirname 'dirname', t2.dirname 'parentname'
FROM Tablename t1
INNER JOIN Tablename t2 ON t1.catid = t2.parentcatid
WHERE t2.catid = 243;

SQL Fiddle Demo

This will give you:

| CATID | DIRNAME | PARENTNAME |
--------------------------------
|   243 | mobiles |      Nokia |

Answer 2

Use a self-join:

SELECT parent.catid, parent.dirname
FROM yourtable AS parent
LEFT JOIN yourtable AS child ON (child.parentcatid = parent.catid)
WHERE child.catid = 243

Answer 3

The literal answer to your question is to use a self-join (as Marc B suggests).

Reading between the lines, I'm assuming that you don't know how deep your hierarchy is going to be in advance - that makes the self join less useful, because if you have to continue to join to get deeper levels of the tree, and you probably need to user outer joins if not all parts of the tree go to the same depth.

The alternative approaches - much more complex, and only do this if my assumption is true - is to use a recursive function , or to represent your data as a nested set .