将多个查询合并为一个查询

Question

I have a simple query:

SELECT t1.tbl, 
       t1.slug
FROM t1
WHERE tags = '%".$tag."%' 

However, I need to augment my results with the data from other tables (t2, t3, t4 and t5). For example, if t1.tbl = 't2' I need to add from:

SELECT t2.title
FROM t2    
WHERE t2.county = '".$county."'

which I could join like this:

LEFT JOIN ON (t1.rid = t2.id)

In each of there tables I'll filter by $county even though the column is named differently.

I've tried something like this:

SELECT t1.tbl, 
       t1.slug
FROM t1 A
LEFT JOIN (
   SELECT title
   FROM t2 B
   WHERE A.tbl = 't2'
) ON (A.rid = B.id)
WHERE A.tags = '%".$tag."%'

Is there a way to combine all there into a single query?

Answer 1

SELECT A.tbl, 
       A.slug,
       COALESCE(B.title, C.title) AS title
FROM t1 A
LEFT JOIN t2 B
  ON A.tbl = 't2' AND A.rid = B.id AND B.county = ?
LEFT JOIN t3 C
  ON A.tbl = 't3' AND A.rid = C.id AND C.region = ?
WHERE A.tags LIKE ?
  AND COALESCE(B.id, C.id) IS NOT NULL;

The last condition is to return only rows from A that have a match among one of the joined tables.

I think that's enough to see the pattern, so you can add more tables.

I urge you to learn to use query parameters instead of concatenating variables directly into your SQL string. It's easier to write the code and more secure from SQL injection vulnerabilities if you use query parameters.