将SQL重写为JOINS而不是子查询

Question

I have some troubles re-writing this query to using joins instead of the two subqueries. I have a problem untangling it, if you know what i mean.

SELECT o.order_id, n.title, c.first_name, t1.name, o.product_id,
    (SELECT ttd2.tid FROM term_data ttd2, term_node ttn2 WHERE ttd2.vid = 5 AND ttn2.nid = p.nid AND ttd2.tid=ttn2.tid) AS tid,
    (SELECT ttd4.name FROM term_data ttd4, term_node ttn4 WHERE ttd4.vid = 8 AND ttn4.nid = p.nid AND ttd4.tid=ttn4.tid) AS month
    FROM orders o, products p, node n, customers c, term_data t1, term_node t2
    WHERE o.product_id = p.nid
    AND p.nid = n.nid
    AND o.customer_email = c.customer_email
    AND t2.tid = t1.tid
    AND t1.vid = 6
    AND n.nid = t2.nid

Can you help? Or give some clues/hints.

Answer 1

Rewrite it using ANSI SQL-92 syntax (ie, using JOIN and ON clauses), and it should be much more clear.

Right now all of your JOIN and WHERE clauses are mixed together, so it is not easy to see the relationships. Subqueries are not necessarily a problem; this should become more clear once the syntax is cleaned up.

Answer 2

As a rule:

Move your SELECT ed columns up to the main SELECT statement, move your FROM clause to a JOIN clause below the original FROM , and your join condition to that line as well. Your WHERE clause can stay as is.

Also as @RedFilter says, use JOIN and ON clauses. I think you are doing cartesians but I'm not sure due to the syntax.

For example (I don't know if this is valid for your table structure since you don't give it):

SELECT o.order_id, n.title, c.first_name, t1.name, o.product_id,ttd2.tid as 'tid', ttd4.name as 'name'
FROM orders o
INNER JOIN products p ON o.product_id = p.nid
INNER JOIN node n ON AND p.nid = n.nid
INNER JOIN customers c ON o.customer_email = c.customer_email
INNER JOIN term_data t1 ON t2.tid = t1.tid
INNER JOIN term_node t2 ON n.nid = t2.nid 
INNER JOIN ...
WHERE n.nid = t2.nid
AND ttd2.vid = 5 
AND ttn2.nid = p.nid 
AND ttd2.tid=ttn2.tid)
AND t1.vid = 6
AND ...

Answer 3

Rewritten to use ANSI-92 SQL and slightly simplified to remove redundant joins, your query should look something like this:

SELECT o.order_id, 
       n.title, 
       c.first_name, 
       tdv6.name, 
       o.product_id,
       tdv5.tid,
       tdv8.name    month
FROM orders o
     join products p           on o.product_id = p.nid
     join node n               on p.nid = n.nid
     join customers c          on o.customer_email = c.customer_email
     join term_node tnv        on n.nid = tn.nid
     join term_data tdv6       on tn.tid = tdv6.tid AND tdv6.vid = 6
     left join term_data tdv5  on tn.tid = tdv5.tid AND tdv5.vid = 5
     left join term_data tdv8  on tn.tid = tdv8.tid AND tdv8.vid = 8