Mysql Query to check 3 tables for an existing row - php

Question

What I want to do is to query three separate tables into one row which is identified by a unique reference. I don't really have full understanding of the Join clause as it seems to require some sort of related data from each table.
I know I can go about this the long way round, but can not afford to lose even a little efficiency. Any help would be greatly appreciated.
Table Structure
package_id int(8),
client_id int(8),
unique reference varchar (40)
Each of the tables have essentially the same structure. I just need to know how to query all three, for 1 row.

Answer 1

If you have few tables that are sharing the same or similar definition, you can use union or union all to treat them as one. This query will return rows from each table having requested reference. I've included OriginTable info in case your code will need to refer to original table for update or something else.
select 'TableA' OriginTable,
package_id,
client_id
from TableA
where reference = ?
union all
select 'TableB' OriginTable,
package_id,
client_id
from TableB
where reference = ?
union all
select 'TableC' OriginTable,
package_id,
client_id
from TableC
where reference = ?
You might extend select list with other columns, provided that they have the same data type, or are implicitly convertible to data type from first select.

Answer 2

Let's say you have 3 tables :
table1, table2 and table3 with structure
package_id int(8),
client_id int(8),
unique reference varchar (40)
Let's assume that column reference is unique key.
Then you can use this:
SELECT t1.exists_row ,t2.exists_row ,t3.exists_row FROM
(
(SELECT COUNT(1) as exists_row FROM table1 t1 WHERE
t1.reference = #reference ) t1,
(SELECT COUNT(1) as exists_row FROM table1 t2 WHERE
t2.reference = #reference ) t2,
(SELECT COUNT(1) as exists_row FROM table1 t3 WHERE
t3.reference = #reference ) t3
) a
;
Replace #reference with actual value of unique key
or when you provide output of
SHOW CREATE TABLE
I can rewrite SQL with actual query

Answer 3

It is entirely possible to create a join between tables using a where clause. In fact this is often what I do as I find it leads to clearer information of what you are actually doing, and if you don't get the results you expect you can debug it bit by bit.
That said however a join is certainly a lot quicker to write!
Please bear in mind I'm a bi rusty on SQL so I may have missed remembered, and I'm not going to include any code as you haven't said what DBMS you are using as they all have slightly different code.
The thing to remember is that the join functions on a column with the same data (and type) within it.
It is much easier if each table has the 'joining' field named the same, then it should be a matter of
join on <nameOfField>
However if you wish to use field that have different names in the different tables you will need to list the fully qualified names. ie tableName.FieldName
If you are having trouble with natural, inner and outer, left and right, you need to think of a venn diagram with the natural being the point of commonality between the tables. If you are using only 2 tables inner and outer are equivalent to left and right (with each table being a single circle in the venn diagram) and left and right being the order of the tables in your list in the main part of your select (the first being the left and the second being the right).
When you add a third table this is where you can select any of the cross over section using these keywords.
Again however I have always found it easier to do a primary select and create a temp table, then perform my next join using this temp table (so effectively only need to use natural or left and right again). Again I find this easier to debug.
The best thing is to experiment and see what you get in return. Without a diagram of your tables this is the best I can offer.
in brief...
nested selects where field = (select from table where field = )
and temp tables
are (I think) easier to debug... but do take more writting !
David.

Answer 4

array_of_tables[]; // contain name of each table
foreach(array_of_tables as $val)
{
$query="select * from `$val` where $condition "; // $conditon
$result=mysqli_query($connection,$query);
$result_row[]=mysqli_fetch_assoc($result); // if only one row going to return form each table
//check resulting array ,for your row
}

Answer 5

SELECT * FROM table1 t1 JOIN table2 t2 ON (t2.unique = t1.unique) JOIN table3 t3 ON (t3.unique = t1.unique) WHERE t1.unique = '?';
You could use a JOIN like this, assuming all three tables have the same unique column.