I have a table taged with two fields sesskey (varchar32 , index) and products (int11), now I have to delete all rows that having group by sesskey count(*) = 1.
I'm trying a fews methods but all fails.
Example:
delete from taged where sesskey in (select sesskey from taged group by sesskey having count(*) = 1)
The sesskey field could not be a primary key because its repeated.
DELETE si
FROM t_session si
JOIN (
SELECT sesskey
FROM t_session so
GROUP BY
sesskey
HAVING COUNT(*) = 1
) q
ON q.sesskey = si.sesskey
You need to have a join here. Using a correlated subquery won't work.
See this article in my blog for more detail:
Keeping rows
Or if you're using an older (pre 4.1) version of MySQL and don't have access to subqueries you need to select your data into a table, then join that table with the original:
CREATE TABLE delete_me_table (sesskey varchar32, cur_total int);
INSERT INTO delete_me_table SELECT sesskey, count(*) as cur_total FROM orig_table
WHERE cur_total = 1 GROUP BY sesskey;
DELETE FROM orig_table INNER JOIN delete_me_table USING (sesskey);
Now you have a table left over named delete_me_table which contains a history of all the rows you deleted. You can use this for archiving, trending, other fun and unusual things to surprise yourself with.
The SubQuery should work
Delete from taged
Where sesskey in
(Select sesskey
From taged
Group by sesskey
Having count(*) = 1)
EDIT: Thanks to #Quassnoi comment below... The above will NOT work in MySql, as MySql restricts referencing the table being updated or deleted from, in a Subquery i you must do the same thing using a Join ...
Related
I table data is like this:
id car_id create_time remark
6c3befd0201a4691 4539196f55b54523986535539ed7beef 2017-07-1 16:42:49 firstcar
769d85b323bb4a1c 4539196f55b54523986535539ed7beef 2017-07-18 16:42:49 secondcar
984660c4189e499 575d90e340d14cf1bef4349b7bb5de9a 2017-07-3 16:42:49 firstjeep
I want to get the newest data. It means if there have two same car_id, I want to get only one according the newest time. How to write?
I try to write this, but I find it may wrong. If the other record may have the same create_time? How to fix that?
SELECT * FROM t_decorate_car
WHERE create_time IN
(SELECT tmptime FROM
(SELECT MAX(create_time),tmptime,car_id
FROM decorate
GROUP BY car_id
) tmp
)
One canonical way to handle this is to join your table to a subquery which finds the latest record for each car_id. This subquery serves as a filter to remove the older records you don't want to see.
SELECT t1.*
FROM t_decorate_car t1
INNER JOIN
(
SELECT car_id, MAX(create_time) AS max_create_time
FROM t_decorate_car
GROUP BY car_id
) t2
ON t1.car_id = t2.car_id AND
t1.create_time = t2.max_create_time
By the way, if you want to continue down your current road, you can also solve this using a correlated subquery:
SELECT t1.*
FROM t_decorate_car t1
WHERE t1.create_time = (SELECT MAX(t2.create_time) FROM t_decorate_car t2
WHERE t2.car_id = t1.car_id)
You were on the right track but you never connected the subquery to the main query using the right WHERE clause.
I have just studied FROM clause and derived tables in mysql and most of the websites provided the examples using SELECT command
Example SELECT * FROM (SELECT * FROM usrs) as u WHERE u.name = 'john'
But when I have tried using delete or update command it does not seem to work.
Example DELETE FROM (SELECT * FROM usrs) as u WHERE u.name = 'john'
1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near (SELECT * FROM usrs) as u WHERE u.name = 'john' at line
UPDATE (SELECT * FROM usrs) as u SET u.lname ='smith' WHERE u.name = 'john'
1288 The target table e of the UPDATE is not updatable
So derived tables does not work with delete or update commands? or is there a way to make it work.
Instead of writing the table name for update and delete I want to write a subquery that gets the records and perform the delete operation on that records? Is that possible in mysql?
UPDATED I have to delete a record and i have three tables, the record may exist in any of the table
My approach delete from first table rows effected? quit: else check second table rows effected? quit : else check third table
But if I use UNION ALL I can do this way
Delete from (select * from tb1 union all select * from tb2 union all select * from tb3) e as e.uname = 'john'
but this query does not seem to work , now could anyone tell me how do i delete or update a record when i have more than one table to search. Any help is greatly appreciated.
You can't directly delete from the subquery, but you can still use it if you'd like, you'll just need to use it in a JOIN:
DELETE usrs
FROM usrs
INNER JOIN (
SELECT * FROM usrs WHERE name = 'john'
) t ON usrs.Id = t.Id
Or you could use IN:
DELETE usrs
WHERE ID IN (
SELECT ID
FROM usrs
WHERE name = 'John'
)
With this said, for this example, I don't know why you'd want a subquery:
DELETE usrs WHERE name = 'John'
Edit base on comments. To delete from multiple tables at the same time, you can either have multiple DELETE statements, or you can use something like the following:
delete t1, t2, t3
from (select 'john' as usr) t
left join t1 on t.usr=t1.usr
left join t2 on t.usr=t2.usr
left join t3 on t.usr=t3.usr
SQL Fiddle Demo
Derived tables exist only for the duration of the parent query they're a member of. Assuming that this syntax and the operations were allowed by MySQL, consider what happens:
a) Your main query starts executing
b) the sub-query executes and returns its results as a temporary table
c) the parent update changes that temporary table
d) the parent query finishes
e) temporary tables are cleaned up and deleted
Essentially you'll have done nothing except waste a bunch of cpu cycles and disk bandwidth.
UPDATE queries DO allow you to join against other tables to use in the WHERE clause, e.g..
UPDATE maintable
LEFT JOIN othertable ON maintable.pk = othertable.fk
SET maintable.somefield='foo'
WHERE othertable.otherfield = 'bar'
I am running a delete which removes all of the duplicates within a table. A duplicate is defined as a row where the tag_id, user_id, and is_self are all the same. My technique here is pretty standard, to preform this delete, since the tags_users table itself needs to be referenced to know if a duplicate exists a temp table is created so that a delete can be preformed from the same table that is being referenced. The problem is that this table is about a million rows so this query takes about an hour to run. I know this is related to the slow speed of defining this temp table and then referencing it as it is un-indexed.
DELETE FROM tags_users WHERE id IN (
SELECT id FROM (
SELECT A.id FROM tags_users as A, tags_users as B WHERE A.id > B.id AND A.user_id = B.user_id AND A.tag_id = B.tag_id AND A.is_self = B.is_self GROUP BY A.id
) temp_dup_delete
);
I have reviewed the explain from this query listed here (Please note I'm on mysql 5.5 so I'm using EXPLAIN SELECT 1 to simulate EXPLAIN DELETE). I think the best possible solution to this is to define an index on the temp table, but I cannot figure out how to do this yet. The crux of my question here is: is there a way to improve the speed of this query considering the way it defines a temp table. Thank you to anyone that can help.
Here is an alternative approach. Use an aggregation query to find the minimum id for each set of key values -- this seems to be the row you want to keep.
Then, use left outer join to match to this table and delete all the rows in the original data that do not match.
delete tu
from tags_users tu left outer join
(select tag_id, user_id, is_self, min(id) as minid
from tags_users
group by tag_id, user_id, is_self
) tui
on tui.id = tu.id
where tui.id is null;
What would be the best way to return one item from each id instead of all of the other items within the table. Currently the query below returns all manufacturers
SELECT m.name
FROM `default_ps_products` p
INNER JOIN `default_ps_products_manufacturers` m ON p.manufacturer_id = m.id
I have solved my question by using the DISTINCT value in my query:
SELECT DISTINCT m.name, m.id
FROM `default_ps_products` p
INNER JOIN `default_ps_products_manufacturers` m ON p.manufacturer_id = m.id
ORDER BY m.name
there are 4 main ways I can think of to delete duplicate rows
method 1
delete all rows bigger than smallest or less than greatest rowid value. Example
delete from tableName a where rowid> (select min(rowid) from tableName b where a.key=b.key and a.key2=b.key2)
method 2
usually faster but you must recreate all indexes, constraints and triggers afterward..
pull all as distinct to new table then drop 1st table and rename new table to old table name
example.
create table t1 as select distinct * from t2; drop table t1; rename t2 to t1;
method 3
delete uing where exists based on rowid. example
delete from tableName a where exists(select 'x' from tableName b where a.key1=b.key1 and a.key2=b.key2 and b.rowid >a.rowid) Note if nulls are on column use nvl on column name.
method 4
collect first row for each key value and delete rows not in this set. Example
delete from tableName a where rowid not in(select min(rowid) from tableName b group by key1, key2)
note that you don't have to use nvl for method 4
Using DISTINCT often is a bad practice. It may be a sing that there is something wrong with your SELECT statement, or your data structure is not normalized.
In your case I would use this (in assumption that default_ps_products_manufacturers has unique records).
SELECT m.id, m.name
FROM default_ps_products_manufacturers m
WHERE EXISTS (SELECT 1 FROM default_ps_products p WHERE p.manufacturer_id = m.id)
Or an equivalent query with IN:
SELECT m.id, m.name
FROM default_ps_products_manufacturers m
WHERE m.id IN (SELECT p.manufacturer_id FROM default_ps_products p)
The only thing - between all possible queries it is better to select the one with the better execution plan. Which may depend on your vendor and/or physical structure, statistics, etc... of your data base.
I think in most cases EXISTS will work better.
I got the table with the fields: id, foreign_key, created, modified
It's a table that logs the changes of a part of my program. What I want is to retrieve the latest logs grouped by the foreign key. How do I do this?
EDIT:
to summarize, how do I order first before I group?
SELECT ... GROUP BY ... HAVING MAX(modified)
Uncorrelated subquery:
SELECT t.*
, a.*
FROM mytable t
INNER JOIN
(select b.foreign_key
, max(b.modified) as max_modified
from mytable b
group by 1) a
ON a.foreign_key = t.foreign_key
AND a.max_modified = t.modified