结合两个SQL查询PHP

Question

Thanks for reading my question. I've been going at this for a day and a half now and I cant seem to get this to work. What I'm trying to do is combine two SQL queries.

The connection to the database is correctly set-up. This is all working. So the queries I'm trying to combine are:

$sql = "SELECT * FROM pf_postmeta WHERE meta_key = '_wp_attached_file' ";
$result = $conn->query($sql);
foreach ($result as $row) {
        // do my stuff
    }
$conn->close();

And

$sql = "SELECT * FROM pf_posts WHERE post_type = 'cases' ";
$result = $conn->query($sql);
foreach ($result as $row) {
        // do my stuff
    }
$conn->close();

Now I've read about JOIN and UNION but I can't get the two queries to work together.

I have tried using UNION ALL to no avail.

$sql = "SELECT *FROM pf_postmeta WHERE meta_key = '_wp_attached_file'
        UNION ALL
        SELECT *FROM pf_posts WHERE post_type = 'cases'";
$result = $conn->query($sql);
foreach ($result as $row) {
        // do stuff
    }
$conn->close();

This doesn't show anything.

UPDATE AS REQUESTED: If I remove the UNION ALL SELECT part my query works. If I run this code in MySQLWorkbench it say's

09:07:30 SELECT * FROM deb100651n2_pf.pf_posts WHERE post_type='cases' UNION ALL SELECT * FROM deb100651n2_pf.pf_postmeta WHERE meta_key = '_wp_attached_file' Error Code: 1222. The used SELECT statements have a different number of columns 0.050 sec

Is there anyone who has a solution form me? This has been bugging me for the past 1,5 day's...

(Note: I'm pretty new to the whole SQL thing, so please be gentle ...)

Again thanks for your time...

Answer 1

To perform a "union query" the number of columns MUST match : plus , the data type of each column must be compatible (eg column 3 of first query is decimal, and column 3 of second query is integer). If either the number of columns is different, or any column is of an incompatible type, the query will produce an SQL error.

So. Here is a very great reason to cease using select * in any production code. You must specify the columns names, making sure both select lists have the same number of columns, and that those columns are compatible.

Answer 2

As far I guessed from what you have stated above there might be column miss match issue in your union query. While using union operator you must remember three things-

• Each SELECT statement within UNION must have the same number of columns
• The columns must also have similar data types
• The columns in each SELECT statement must also be in the same order

Therefore if you perform a SELECT * .. operation on both table then your pf_postmeta and pf_post table must have same number of column having similar data types and ordering. You may try SELECT id from pf_postmeta UNION ALL SELECT id from pf_post for clearing your concept how union works assuming there is id column at both table.

Answer 3

why to make so complicate make it simple. Try it:

SELECT *  FROM pf_postmeta 
WHERE meta_key = '_wp_attached_file'
UNION
SELECT * FROM pf_posts
WHERE post_type = 'cases'