How to get last inserted rows in MySQL from different tables? - mysql

Question

I have a lot of different tables in my database, and I need somehow to get last inserted rows from those tables. Like social networks feeds. Also, those tables are have not random, but unknown names, because they all generated by users.
In example:
I have tables: A,B,C and D with 5k rows in each table.
I need somehow to get last rows from those tables and make it ordered by id, like we do in a simple query: "SELECT * FROM table A ORDER BY id DESC", but I'm looking for something like: "SELECT * FROM A,B,C,D ORDER BY id DESC".
Tables have same structure.

Answer 1

You can use union and order by if your tables have the same structure. Something like:
select *
from (
select * from A
union all
select * from B
union all
select * from C
) order by id desc
If the tables don't have the same structure then you cannot select * from all and order them and you might want to do two queries. First would be:
select id, tableName
from (
select id, 'tableA' as tableName from A
union all
select id, 'tableB' as tableName from B
union all
select id, 'tableC' as tableName from C
) order by id desc
Which will give you the last IDs and the tables where they are inserted. And then you need to get the rows from each respective table.
With pure Mysql it will be a bit hard. You can select the table names like:
SELECT table_name FROM information_schema.tables; but still how to use them in the statement? You will need to generate it dynamically
A procedure to generate the query dynamically can be something like ( I haven't tested it but I believe with some debugging it should work) :
DELIMITER $$
CREATE PROCEDURE buildQuery (OUT v_query VARCHAR)
BEGIN
DECLARE v_finished INTEGER DEFAULT 0;
DECLARE v_table_count INTEGER DEFAULT 0;
DECLARE v_table varchar(100) DEFAULT "";
-- declare cursor for tables (this is for all tables but can be changed)
DEClARE table_cursor CURSOR FOR
SELECT table_name FROM information_schema.tables;
-- declare NOT FOUND handler
DECLARE CONTINUE HANDLER
FOR NOT FOUND SET v_finished = 1;
OPEN table_cursor;
SET v_query="select * from ( ";
get_table: LOOP
FETCH table_cursor INTO v_table;
SET v_table_count = v_table_count + 1;
IF v_finished = 1 THEN
LEAVE get_table;
END IF;
if v_table_count>1 THEN
CONCAT(vquery, " UNION ALL ")
END IF;
SET v_query = CONCAT(vquery," select * from ", v_table );
END LOOP get_table;
SET v_query = CONCAT(vquery," ) order by id desc " );
-- here v_query should be the created query with UNION_ALL
CLOSE table_cursor;
SELECT #v_query;
END$$
DELIMITER ;

Answer 2

If each table's id is counted seperatly you can't order by ID, so you'll need to calculate a global id and use it on all of your tables.
You can do it as follows:
Assuming you have 2 tables A,B:
Create Table A(id int NOT NULL auto_increment, name varchar(max), value varchar(max), PRIMARY_KEY(id));
Create Table B(id int NOT NULL auto_increment, name varchar(max), value varchar(max), PRIMARY_KEY(id));
Add another table IDS with id as auto increment primary key.
Create table IDS (id int NOT NULL auto_increment, ts Timestamp default CURRENT_TIMESTAMP, PRIMARY_KEY(id));
For all your tables id column should use now the id from the IDS table as foreign key instead of auto increment.
Create Table A(id int NOT NULL auto_increment, name varchar(max), value varchar(max), PRIMARY_KEY(id),CONSTRAINT fk_A_id FOREIGN KEY(id) REFERENCE IDS(id) ON DELETE CASCADE ON UPDATE CASCADE);
Create Table B(id int NOT NULL auto_increment, name varchar(max), value varchar(max), PRIMARY_KEY(id),CONSTRAINT fk_A_id FOREIGN KEY(id) REFERENCE IDS(id) ON DELETE CASCADE ON UPDATE CASCADE);
for each table add before insert trigger, the trigger should first insert row to the IDS table and insert the LAST_INSERT_ID the table.
Create TRIGGER befor_insert_A BEFORE INSERT On A
FOR EACH ROW
BEGIN
insert into IDS() values ();
set new.id = LAST_INSERT_ID();
END
Create TRIGGER befor_insert_B BEFORE INSERT On B
FOR EACH ROW
BEGIN
insert into IDS() values ();
set new.id = LAST_INSERT_ID();
END
Now you can create view from all tables with union all, the rows of v can be sorted now by the id and give the cronlogic order of insertion.
Create view V AS select * from A UNION ALL select * from B
For example you can query on V the latest 10 ids:
select * from V Order by id desc LIMIT 10
Other option is to add timestamp for each table and sort the view by the timestamp.

Answer 3

Hi are you looking for this? However the id is not a good column to see the last updated among different tables.
select *
from A
join B
on 1=1
join C
on 1=1
join D
on 1=1
order by A.id desc