Question

I want to join these two table but the join key of the second table is in a query string,

page table,

page_id     url
1           a
2           c
3           d

system table,

system_id       query
1               page_id=1&content=on&image=on
2               type=post&page_id=2&content=on

as you can see that page_id is part of the query string in system table.

so how can I join them like the standard joining table method below?

SELECT*
FROM page AS p

LEFT JOIN system AS s
ON p.page_id = s.page_id

EDIT:

I def can change the system table into something like this,

system_id       page_id    query
1               1           page_id=1&content=on&image=on
2               2           type=post&page_id=2&content=on
3               NULL        type=page

But the reason why I don’t want to do this is that the page_id is no need for many certain records. I don’t want make a column with too many null.

Answer 1

I guess you wanted something like this (MSSQL!):

---

DECLARE @query VARCHAR(50)
DECLARE @Lenght INT  
DECLARE @PageID INT 

SET @query = '4kkhknmnkpage_id=231&content=on&image=on'
SET @Lenght = PATINDEX('%&%', substring(@query,PATINDEX('%page_id=%', @query),50)) - 9
SET @PageID = CAST(SUBSTRING(@query,PATINDEX('%page_id=%', @query) + 8,@Lenght) AS INT)

SELECT @PageID -- you can do as you please now :)

---

OR:

SELECT*
FROM page AS p
LEFT JOIN (SELECT CAST(SUBSTRING(query,PATINDEX('%page_id=%', query) + 8,(PATINDEX('%&%', substring(query,PATINDEX('%page_id=%', query),50)) - 9)) AS INT) AS page_id
                FROM system) AS s
ON p.page_id = s.page_id 

-- Do as you please again :) 

---

I guess what you really wanted was something like this (MYSQL!):

---

SET @query := '4kkhknmnkpage_id=231&content=on&image=on';
SET @Lenght := POSITION('&' IN (SUBSTR(@query,POSITION('page_id=' IN @query),50))) - 9;
SET @PageID := CAST(SUBSTR(@query,POSITION('page_id=' IN @query) + 8,@Lenght) AS  SIGNED );

SELECT @PageID

---

OR

---

SELECT*
FROM page AS p
LEFT JOIN (SELECT CAST(SUBSTR(query,POSITION('page_id=' IN query) + 8,(POSITION('&' IN (SUBSTR(query,POSITION('page_id=' IN query),50))) - 9)) AS  SIGNED) AS pageID
           FROM system) AS s
ON p.page_id = s.pageID