Select all record from all the tables, every derived table must have its own alias - sql

Question

I'm working on a e-learning project in which there is a table named chapter in which there is a column named question_table this is table in which the specific chapter's questions are added.
Now the problem is I want to display all the question from all the chapter for this I used following sql query
SELECT * FROM (SELECT `question_table` FROM `chapter`)
but it doesn't work and gives the error:
"Every derived table must have its own alias".
Note: I want to do it using SQL not PHP.

Answer 1

Firstly, I think you would be better redesigning your database. Multiple tables of the same structure holding the same data are generally not a good idea.
However what you require is possible using a MySQL procedure to build up some dynamic SQL and then execute it, returning the resulting data.
A procedure as follows could be used to do this:-
DROP PROCEDURE IF EXISTS dynamic;
delimiter //
CREATE PROCEDURE dynamic()
BEGIN
DECLARE question_table_value VARCHAR(25);
DECLARE b INT DEFAULT 0;
DECLARE c TEXT DEFAULT '';
DECLARE cur1 CURSOR FOR SELECT `question_table` FROM `chapter`;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET b = 1;
OPEN cur1;
SET b = 0;
WHILE b = 0 DO
FETCH cur1 INTO question_table_value;
IF b = 0 THEN
IF c = '' THEN
SET c = CONCAT('SELECT * FROM `',question_table_value, '`');
ELSE
SET c = CONCAT(c, ' UNION SELECT * FROM `',question_table_value, '`');
END IF;
END IF;
END WHILE;
CLOSE cur1;
SET #stmt1 := c;
PREPARE stmt FROM #stmt1;
EXECUTE stmt;
END
This is creating a procedure called dynamic. This takes no parameters. It sets up a cursor to read the question_table column values from the chapter table. It looks around the results from that, building up a string which contains the SQL, which is a SELECT from each table with the results UNIONed together. This is then PREPAREd and executed. The procedure will return the result set from the SQL executed by default.
You can call this to return the results using:-
CALL dynamic()
Down side is that this isn't going to give nice results if there are no rows to return and they are not that easy to maintain or debug with the normal tools developers have. Added to which very few people have any real stored procedure skills to maintain it in future.

Answer 2

In MySQL you must give every subquery ("derived table") an alias:
SELECT * FROM (SELECT question_table FROM chapter) t --notice the alias "t"

Answer 3

The derived table here is the result of the (SELECT ...). You need to give it an alias, like so:
SELECT * FROM (SELECT question_table FROM chapter) X;
Edit, re dynamic tables
If you know all the tables in advance, you can union them, i.e.:
SELECT * FROM
(
SELECT Col1, Col2, ...
FROM Chapter1
UNION
SELECT Col1, Col2, ...
FROM Chapter2
UNION
...
) X;
SqlFiddle here
To do this solution generically, you'll need to use dynamic sql to achieve your goal.
In general however, this is indicative of a smell in your table design - your chapter data should really be in one table, and e.g. classified by the chapter id.
If you do need to shard data for scale or performance reasons, the typical mechanism for doing this is to span multiple databases, not tables in the same database. MySql can handle large numbers of rows per table, and performance won't be an issue if the table is indexed appropriately.