I do not understand MySQL delete when I need to delete data in a table with data from another table that depend on it.
For example if I want to delete a data in table 'factory', all data at table 'room' that depends on data at table 'factory is also deleted'.
Fac_ID is the primary key in 'factory' and foreign key in 'room'
below is my SQL code.
DELETE * FROM factory
LEFT JOIN room ON room.Fac_ID = factory.Fac_ID
WHERE factory.Fac_ID = :Fac_ID
Can any one help me?
I think you need a separate delete for this.
First is to delete foreign data
delete from room where Fac_ID = :Fac_ID
Then delete primary data
delete from factory where Fac_ID = :Fac_ID
Unless your table design is ON DELETE CASCADE (supported only in INNODB), you only need to delete the primary data
There is a small mistake in your query because of which I think you are facing problem.
As per my understanding you have some records in Main table and some records in refrenced table. There are some case for which main table has some id but there is not entry in refrence table for that id. And for handling that case you applied left join.
But in your query you wrote reference table on left so basically it is taking all of the record from reference table which is kind of inner join in this case.
So for correcting this you need to interchange the key id pass in your query or you may use right join to select all records from main table.
DELETE * FROM factory
LEFT JOIN room ON factory.Fac_ID = room.Fac_ID --- here you applied left join
WHERE factory.Fac_ID = :Fac_ID
MySQL allows you to delete rows from multiple tables at the same time. The syntax is:
DELETE f, r
FROM factory f LEFT JOIN
room r
ON r.Fac_ID = f.Fac_ID
WHERE f.Fac_ID = :Fac_ID;
However, this is better set up as a cascading delete foreign key relationship between the two tables.
Related
table 1 is called (athlete) and table2 is called (training_session.id) the primary key to table 1 is ID, and the table 2 has the primary key Athelete_id
I want to delete a person from my database by using his name, which I've called "Pet". However, he is also connected to another table which stores his training session. So (ID 1) on table 1 is connected to table 2 (athlete id1)
I struggle a lot, I try using INNER JOIN.
DELETE athlete,training_session FROM athlete
INNER JOIN
training_session ON training_session.id = athlete.name
WHERE
athlete.name = "Pet;
I have something wrong with my syntax, is it correct to use Inner Join or have I misunderstood
You should have set up foreign key constraints with Cascade deletions to simplify the logic and all you would have needed than was to delete from athlete. So I would suggest you add it.
For more info you can take a look at:
http://www.mysqltutorial.org/mysql-on-delete-cascade/
I have a address table which is referenced from 6 other tables (sometimes multiple tables). Some of those tables have around half a million records (and the address table around 750000 records). I want to have a periodical query running which deletes all records that are not referenced from any of the tables.
The following sub-queries is not a option, because the query never finishes - the scope is too big.
delete from address where address_id not in (select ...)
and not in (select ...) and not in (select ...) ...
What I was hoping was that I could use the foreign key constraint and I could simply delete all records for which the foreign key constraint does not stop me (because there is no reference to the table). I could not find a way to do this (or is there?). Anybody another good idea to tackle this problem?
You can try this ways
DELETE
address
FROM
address
LEFT JOIN other_table ON (address.id = other_table.ref_field)
LEFT JOIN other_table ON (address.id = other_table2.ref_field)
WHERE
other_table.id IS NULL AND other_table2.id IS NULL
OR
DELETE
FROM address A
WHERE NOT EXISTS (
SELECT 1
FROM other_table B
WHERE B.a_key = A.id
)
I always use this:
DELETE FROM table WHERE id NOT IN (SELECT id FROM OTHER table)
I'd do this by first creating a TEMPORARY TABLE (t) that is a UNION of the IDs in the 6 referencing tables, then run:
DELETE x FROM x LEFT JOIN t USING (ID) WHERE x.ID IS NULL;
Where x is the address table.
See 'Multiple-table syntax' here:
http://dev.mysql.com/doc/refman/5.0/en/delete.html
Obviously, your temporary table should have its PRIMARY KEY on ID. It may take some time to query and join, but I can't see a way round it. It should be optimized, unlike the multiple sub-query version.
I currently have 2 tables:
CrawlData
id (autoincrement), Source, Destination, and some more columns
Nodes
id (autoincrement), URL
The Nodes table contains the distinct Source values from CrawlData
Now I would like to have a table that is a kind of look up table that contains the ID's from Nodes instead of the texts in Source and Destination from CrawlData
I can get all the ID's with a Select query using Join on the URL=Source and URL = Destination, but don't know how to combine these and then also to get them in a new table Edges with 2 columns:
SourceNode (= id from Nodes where CrawlData.Source = URL)
DestinationNode (= id from Nodes where CrawlData.Destination = URL)
You can INSERT the records returned by SELECT statement using INSERT INTO...SELECT statement.
INSERT INTO Edges(SourceNode, DestinationNode)
SELECT b.ID SourceNode,
c.ID DestinationNode
FROM CrawlData a
INNER JOIN Nodes b
ON a.Source = b.URL
INNER JOIN Nodes c
ON a.Destination = c.URL
To further gain more knowledge about joins, kindly visit the link below:
Visual Representation of SQL Joins
For faster execution, execute the following statements to add INDEX on the columns to avoid FULL TABLE SCAN which could be slow if doing on large RDBMS.
ALTER TABLE Nodes ADD INDEX (URL);
if happens that all values of Source and Destination column are present on Nodes.URL, declare these columns as foreign keys,
ALTER TABLE CrawlData
ADD CONSTRAINT cd_fk1 FOREIGN KEY (Source) REFERENCES Nodes(URL)
ALTER TABLE CrawlData
ADD CONSTRAINT cd_fk2 FOREIGN KEY (Destination) REFERENCES Nodes(URL)
otherwise, add normal index on them
ALTER TABLE CrawlData ADD INDEX (Source);
ALTER TABLE CrawlData ADD INDEX (Destination);
You can join twice to the Nodes table. Once, use the Source to join to URLK. The next time use the Destination.
Conceptually, it is like using two copies of the Nodes table, each with a different name (say "S" and "D"). You get:
select S.ID As SOURCE_ID, D.ID As DEST_ID
from CrawlData
join Nodes S on Source = S.URL
join Nodes D on Destination = D.URL
I want to create a unique combokey on Site and ASIN and remove duplicates while creating
so far I have
ALTER TABLE Products ADD UNIQUE (Site,ASIN)
I figure there needs to be an "On Duplicate" but I cant find the right thing to do within http://dev.mysql.com/doc/refman/5.0/en/alter-table.html .. does anyone have a suggestion?
I don't think there is such a thing as ON DUPLICATE when adding a UNIQUE KEY. So you need to find a workaround. Here is a suggestion:
1- Identify the duplicate rows and store the values in a temp table.
CREATE TEMPORARY TABLE Duplicates
(SELECT
Site, ASIN
FROM
Products
GROUP BY
Site, ASIN
HAVING
COUNT(*) > 1
);
2- join the temp table with the original table and delete the duplicate rows that you don't need anymore. (from the comments I think you want to remove the ones with SKU IS NULL)
DELETE p
FROM
Products AS p
INNER JOIN
Duplicates AS d
ON
d.Site = p.Site and d.ASIN = p.ASIN
WHERE
p.SKU IS NULL;
3- create your unique key:
ALTER TABLE Products ADD UNIQUE KEY (Site, ASIN);
Here is a fiddle I created.
There are 2 tables, spawnlist and npc, and I need to delete data from spawnlsit.
npc_templateid = n.idTemplate is the only thing that "connect" the tables.
I have tried this script but it doesn't work.
I have tried this:
DELETE s FROM spawnlist s
INNER JOIN npc n ON s.npc_templateid = n.idTemplate
WHERE (n.type = "monster");
Add .* to s in your first line.
Try:
DELETE s.* FROM spawnlist s
INNER JOIN npc n ON s.npc_templateid = n.idTemplate
WHERE (n.type = "monster");
If the database is InnoDB then it might be a better idea to use foreign keys and cascade on delete, this would do what you want and also result in no redundant data being stored.
For this example however I don't think you need the first s:
DELETE s
FROM spawnlist AS s
INNER JOIN npc AS n ON s.npc_templateid = n.idTemplate
WHERE n.type = "monster";
It might be a better idea to select the rows before deleting so you are sure your deleting what you wish to:
SELECT * FROM spawnlist
INNER JOIN npc ON spawnlist.npc_templateid = npc.idTemplate
WHERE npc.type = "monster";
You can also check the MySQL delete syntax here: http://dev.mysql.com/doc/refman/5.0/en/delete.html
if the database is InnoDB you dont need to do joins in deletion. only
DELETE FROM spawnlist WHERE spawnlist.type = "monster";
can be used to delete the all the records that linked with foreign keys in other tables, to do that you have to first linked your tables in design time.
CREATE TABLE IF NOT EXIST spawnlist (
npc_templateid VARCHAR(20) NOT NULL PRIMARY KEY
)ENGINE=InnoDB;
CREATE TABLE IF NOT EXIST npc (
idTemplate VARCHAR(20) NOT NULL,
FOREIGN KEY (idTemplate) REFERENCES spawnlist(npc_templateid) ON DELETE CASCADE
)ENGINE=InnoDB;
if you uses MyISAM you can delete records joining like this
DELETE a,b
FROM `spawnlist` a
JOIN `npc` b
ON a.`npc_templateid` = b.`idTemplate`
WHERE a.`type` = 'monster';
in first line i have initialized the two temp tables for delet the record,
in second line i have assigned the existance table to both a and b but here i have linked both tables together with join keyword,
and i have matched the primary and foreign key for both tables that make link,
in last line i have filtered the record by field to delete.