I've got a table with multiple duplicated entries in a column and I want to put these entries in a new table and connect these tables with a foreign key in the initial table.
Old Table:
table 1
| id | name | medium |
| 0 | xy | a |
| 1 | xz | b |
| 2 | yz | a |
new Table:
table 1 table2
| id | name | medium | | id | name |
| 0 | xy | 0 | | 0 | a |
| 1 | xz | 1 | | 1 | b |
| 2 | yz | 0 |
With CREATE ... SELECT I have a good tool to create a new table from the results of a query but I don't know how to change the entries from table1.medium to a foreign key based on the comparison to table2.medium. Is there any chance to do that?
You can do:
-- Create the second table
create table table2 (
id int primary key,
name varchar(255) unique
);
-- Insert rows into it
insert into table2
select row_number() over (order by medium), medium
from original
group by medium;
-- Use JOIN to create the first table and populate with the right ids
create table table1 as
select o.id, o.name, t2.id as medium
from original o join
table2 t2
on t2.medium = o.name;
Here is a db<>fiddle.
You can turn the id in table2 then into an auto_increment column:
alter table table2 modify column id int auto_increment;
It sounds like you can create Table2 but are having trouble changing Table1, updating the column type from char to int and creating a foreign key.
Easiest would be to rename Table1 to Table3 then create Table1 as you want, insert the data, then drop Table3.
If you need to modify the existing table, it will be a multi-step process.
Add new column 'medium_old' and copy existing values from medium
Drop the column 'medium' with the char type
Add the column 'medium' with the new int type
Update the new 'medium' values to the key from Table2 based on values in 'medium_old'
Add the foreign key constraint to 'medium'
There are two ways of doing it:
You recreate table1 with the new values in the medium column. Then you drop the old table1 and rename the new table. I would use this approach if medium column not only needs new values, but also needs to be converted to a new data type - this seems to be the case in the question. You already figured this approach out with create table ... as ...
Using multi-table update syntax, you can easily update a field in a table based on a field in another table. I would use this approach if you do not need to change the data type of the medium column. See this SO question on how to do this update.
If you are running mysql version 8.0 or above you can do below and use window function ROW_NUMBER.
Source
CREATE TABLE TABLE2
AS
SELECT ID,NAME(
SELECT ID, MEDIUM AS NAME,ROW_NUMBER()OVER(PARTITION BY MEDIUM ORDER BY ID) AS ROWW FROM TABLE1)
WHERE ROWW=1;
-- Adds a new column into your TABLE1 for MEDIUM's int values
ALTER TABLE TABLE1 ADD COLUMN MEDIUMINT INT;
-- Update MEDIUMINT according to your TABLE2 values.
UPDATE TABLE1 S1
JOIN TABLE2 S2 ON TABLE1.MEDIUM=S2.NAME
SET S1.MEDIUMINT=S2.ID;
-- Drops MEDIUM column from TABLE1
ALTER TABLE TABLE1 DROP COLUMN MEDIUM;
-- Rename MEDIUMINT column to MEDIUM
ALTER TABLE TABLE1 RENAME COLUMN MEDIUMINT TO MEDIUM;
I will not go deep in details.
I have Java program which is creating tables in Database. The thing is that when table is created one of the fields get default value. So when I am adding elements to this field I want to select default value. Now loading of all element's ID is:
SELECT distinct(codeKind) FROM [table_name];
I want to be something like this:
SELECT [default value of column codeKind] from [table_name];
I checked many of answer of other similiar questions but none of them is OK.
Thanks.
I also find solution:
select COLUMN_DEFAULT
from INFORMATION_SCHEMA.COLUMNS
where TABLE_SCHEMA='my_db' and TABLE_NAME='my_table' and COLUMN_NAME='my_column'
Maybe this
drop table if exists t;
create table t
(id int auto_increment primary key,size int, sizecaption1 int default 10, sizecaption2 int, sizecaption3 int);
insert into t (id,size) values
(1,22);
insert into t values
(2,22,22,22,22),
(3,22,22,30,20),
(4,22,1,2,3);
select * from t where sizecaption1 =
(
select column_default from information_schema.columns
where table_name = 't' and table_schema = 'sandbox' and column_default is not null
) ;
+----+------+--------------+--------------+--------------+
| id | size | sizecaption1 | sizecaption2 | sizecaption3 |
+----+------+--------------+--------------+--------------+
| 1 | 22 | 10 | NULL | NULL |
+----+------+--------------+--------------+--------------+
1 row in set (0.02 sec)
use coalesce
select coalesce(codeKind,<def_value>)
Alternatively you can set a fixed value for table's creation DDL such as
create table my_table ( my_column default 0, col2 int .... ) , if you leave the column value as blank in the insert statement, then the value is automatically populated as zero.
TABLE 1
+----+-------+-------+-------+
| uid | color | brand | model |
+----+-------+-------+-------+
| 10 | 1 | 2 | 1 |
+----+-------+-------+-------+
TABLE 2
+----+-------+-------+-------+
| uid | quantity |model |color|
+----+-------+-------+-------+
| 25 | 2 | 2 | 1 |
+----+-------+-------+-------+
I have many tables like this where the uid column is present in every table.I have a value in a variable, say var1=25. I want to check whether var1 value matches with any of the uid value of any table.If it matches I want to print the table name. Can anyone help me with this?
I tried doing this and I found
SELECT `COLUMN_NAME`
FROM `INFORMATION_SCHEMA`.`COLUMNS`
WHERE `TABLE_SCHEMA`='yourdatabasename'
AND `TABLE_NAME`='yourtablename';
But this is not giving what I want since I want to select all the tables in a database irrespective of the table name.If in future any table is added then it should also get selected.
At first, information_schema table doesn't have specific tuple data.
I suggest you to consider different design.
A. Make a meta table and use triggers(attached to base tables) to maintain meta table.
CREATE TABLE meta_table (
id INT AUTO_INCREMENT,
uid INT,
table_name VARCHAR(50)
);
# When you need to add new table (table 3)
CREATE TABLE table_3 (
uid INT,
field1 INT,
field2 INT,
field3
);
DELIMITER $$
CREATE TRIGGER table_3_insert
AFTER INSERT ON table_3
FOR EACH ROW
BEGIN
INSERT INTO meta_table (uid, table_name)
VALUE (NEW.uid, "table_3");
END$$
DELIMITER ;
# If data in `table_3` might be changed or deleted,
# then create trigger for `delete` and `update`
B. Use only one table with unstructured field and parse data field in your application
CREATE TABLE table (
uid INT,
table_type INT,
data VARCHAR(255)
);
INSERT INTO table (10, 1, '{"color":1,"brand":2,"model":1}');
INSERT INTO table (10, 2, '{"quantity":2,"model":2,"color":1}');
As you mentioned "any table can be added" often, I strongly recommend B solution. It is not good design that changing schema(creating table) often.
I'm having a column called course in my table which as rows lyk
row1 : course:1|course:2|course:3
row2 : Course:2|course:4|NUll
now i have to eliminate the filter '|' in the rows and store the distinct value in another table .. how ll i do it..??
using SP
I'm excepting the output column to be like :
Course:1
Course:2
Course:3
Course:4
Thanks in advance
Try this
create table table1(A varchar(50),B varchar(50),C varchar(50));
insert into table1 values('course:1','course:2','course:3');
insert into table1 values('course:2','course:4',null);
SQL Query
Select Distinct value
From table1
unpivot
(
value
For Cource in (A, B, C)
) U;
Fiddle Demo
Output:
+-----------+
| VALUE |
+-----------+
| course:1 |
| course:2 |
| course:3 |
| course:4 |
+-----------+
Try this function to split row value, look at this link
Then if your data like this:
create table table1 (course varchar(1000))
insert into table1 ( course) values ('course:1|course:2|course:3'),('course:2|course:4|NULL')
This query using function to split string, and you can put this code into Stored Procedure
select ROW_NUMBER()over(order by course)as Number,*
into #temp
from table1
declare #flag int, #count int
select #count=COUNT(''), #flag=1 from table1
create table #result(items varchar(100))
while #flag<=#count
begin
declare #row varchar(1000)
select #row=course from #temp where Number=#flag
insert into #result
select Items from [dbo].[Split](#row,'|')
where Items<>'NULL'
set #flag=#flag+1
end
select distinct * from #result
drop table #result
drop table #temp
Output:
+---------+
|Items |
+---------+
|course:1 |
|course:2 |
|course:3 |
|course:4 |
+---------+
I need to DELETE duplicated rows for specified sid on a MySQL table.
How can I do this with an SQL query?
DELETE (DUPLICATED TITLES) FROM table WHERE SID = "1"
Something like this, but I don't know how to do it.
This removes duplicates in place, without making a new table.
ALTER IGNORE TABLE `table_name` ADD UNIQUE (title, SID)
Note: This only works well if index fits in memory.
Suppose you have a table employee, with the following columns:
employee (first_name, last_name, start_date)
In order to delete the rows with a duplicate first_name column:
delete
from employee using employee,
employee e1
where employee.id > e1.id
and employee.first_name = e1.first_name
Deleting duplicate rows in MySQL in-place, (Assuming you have a timestamp col to sort by) walkthrough:
Create the table and insert some rows:
create table penguins(foo int, bar varchar(15), baz datetime);
insert into penguins values(1, 'skipper', now());
insert into penguins values(1, 'skipper', now());
insert into penguins values(3, 'kowalski', now());
insert into penguins values(3, 'kowalski', now());
insert into penguins values(3, 'kowalski', now());
insert into penguins values(4, 'rico', now());
select * from penguins;
+------+----------+---------------------+
| foo | bar | baz |
+------+----------+---------------------+
| 1 | skipper | 2014-08-25 14:21:54 |
| 1 | skipper | 2014-08-25 14:21:59 |
| 3 | kowalski | 2014-08-25 14:22:09 |
| 3 | kowalski | 2014-08-25 14:22:13 |
| 3 | kowalski | 2014-08-25 14:22:15 |
| 4 | rico | 2014-08-25 14:22:22 |
+------+----------+---------------------+
6 rows in set (0.00 sec)
Remove the duplicates in place:
delete a
from penguins a
left join(
select max(baz) maxtimestamp, foo, bar
from penguins
group by foo, bar) b
on a.baz = maxtimestamp and
a.foo = b.foo and
a.bar = b.bar
where b.maxtimestamp IS NULL;
Query OK, 3 rows affected (0.01 sec)
select * from penguins;
+------+----------+---------------------+
| foo | bar | baz |
+------+----------+---------------------+
| 1 | skipper | 2014-08-25 14:21:59 |
| 3 | kowalski | 2014-08-25 14:22:15 |
| 4 | rico | 2014-08-25 14:22:22 |
+------+----------+---------------------+
3 rows in set (0.00 sec)
You're done, duplicate rows are removed, last one by timestamp is kept.
For those of you without a timestamp or unique column.
You don't have a timestamp or a unique index column to sort by? You're living in a state of degeneracy. You'll have to do additional steps to delete duplicate rows.
create the penguins table and add some rows
create table penguins(foo int, bar varchar(15));
insert into penguins values(1, 'skipper');
insert into penguins values(1, 'skipper');
insert into penguins values(3, 'kowalski');
insert into penguins values(3, 'kowalski');
insert into penguins values(3, 'kowalski');
insert into penguins values(4, 'rico');
select * from penguins;
# +------+----------+
# | foo | bar |
# +------+----------+
# | 1 | skipper |
# | 1 | skipper |
# | 3 | kowalski |
# | 3 | kowalski |
# | 3 | kowalski |
# | 4 | rico |
# +------+----------+
make a clone of the first table and copy into it.
drop table if exists penguins_copy;
create table penguins_copy as ( SELECT foo, bar FROM penguins );
#add an autoincrementing primary key:
ALTER TABLE penguins_copy ADD moo int AUTO_INCREMENT PRIMARY KEY first;
select * from penguins_copy;
# +-----+------+----------+
# | moo | foo | bar |
# +-----+------+----------+
# | 1 | 1 | skipper |
# | 2 | 1 | skipper |
# | 3 | 3 | kowalski |
# | 4 | 3 | kowalski |
# | 5 | 3 | kowalski |
# | 6 | 4 | rico |
# +-----+------+----------+
The max aggregate operates upon the new moo index:
delete a from penguins_copy a left join(
select max(moo) myindex, foo, bar
from penguins_copy
group by foo, bar) b
on a.moo = b.myindex and
a.foo = b.foo and
a.bar = b.bar
where b.myindex IS NULL;
#drop the extra column on the copied table
alter table penguins_copy drop moo;
select * from penguins_copy;
#drop the first table and put the copy table back:
drop table penguins;
create table penguins select * from penguins_copy;
observe and cleanup
drop table penguins_copy;
select * from penguins;
+------+----------+
| foo | bar |
+------+----------+
| 1 | skipper |
| 3 | kowalski |
| 4 | rico |
+------+----------+
Elapsed: 1458.359 milliseconds
What's that big SQL delete statement doing?
Table penguins with alias 'a' is left joined on a subset of table penguins called alias 'b'. The right hand table 'b' which is a subset finds the max timestamp [ or max moo ] grouped by columns foo and bar. This is matched to left hand table 'a'. (foo,bar,baz) on left has every row in the table. The right hand subset 'b' has a (maxtimestamp,foo,bar) which is matched to left only on the one that IS the max.
Every row that is not that max has value maxtimestamp of NULL. Filter down on those NULL rows and you have a set of all rows grouped by foo and bar that isn't the latest timestamp baz. Delete those ones.
Make a backup of the table before you run this.
Prevent this problem from ever happening again on this table:
If you got this to work, and it put out your "duplicate row" fire. Great. Now define a new composite unique key on your table (on those two columns) to prevent more duplicates from being added in the first place.
Like a good immune system, the bad rows shouldn't even be allowed in to the table at the time of insert. Later on all those programs adding duplicates will broadcast their protest, and when you fix them, this issue never comes up again.
Following remove duplicates for all SID-s, not only single one.
With temp table
CREATE TABLE table_temp AS
SELECT * FROM table GROUP BY title, SID;
DROP TABLE table;
RENAME TABLE table_temp TO table;
Since temp_table is freshly created it has no indexes. You'll need to recreate them after removing duplicates. You can check what indexes you have in the table with SHOW INDEXES IN table
Without temp table:
DELETE FROM `table` WHERE id IN (
SELECT all_duplicates.id FROM (
SELECT id FROM `table` WHERE (`title`, `SID`) IN (
SELECT `title`, `SID` FROM `table` GROUP BY `title`, `SID` having count(*) > 1
)
) AS all_duplicates
LEFT JOIN (
SELECT id FROM `table` GROUP BY `title`, `SID` having count(*) > 1
) AS grouped_duplicates
ON all_duplicates.id = grouped_duplicates.id
WHERE grouped_duplicates.id IS NULL
)
After running into this issue myself, on a huge database, I wasn't completely impressed with the performance of any of the other answers. I want to keep only the latest duplicate row, and delete the rest.
In a one-query statement, without a temp table, this worked best for me,
DELETE e.*
FROM employee e
WHERE id IN
(SELECT id
FROM (SELECT MIN(id) as id
FROM employee e2
GROUP BY first_name, last_name
HAVING COUNT(*) > 1) x);
The only caveat is that I have to run the query multiple times, but even with that, I found it worked better for me than the other options.
This always seems to work for me:
CREATE TABLE NoDupeTable LIKE DupeTable;
INSERT NoDupeTable SELECT * FROM DupeTable group by CommonField1,CommonFieldN;
Which keeps the lowest ID on each of the dupes and the rest of the non-dupe records.
I've also taken to doing the following so that the dupe issue no longer occurs after the removal:
CREATE TABLE NoDupeTable LIKE DupeTable;
Alter table NoDupeTable Add Unique `Unique` (CommonField1,CommonField2);
INSERT IGNORE NoDupeTable SELECT * FROM DupeTable;
In other words, I create a duplicate of the first table, add a unique index on the fields I don't want duplicates of, and then do an Insert IGNORE which has the advantage of not failing as a normal Insert would the first time it tried to add a duplicate record based on the two fields and rather ignores any such records.
Moving fwd it becomes impossible to create any duplicate records based on those two fields.
The following works for all tables
CREATE TABLE `noDup` LIKE `Dup` ;
INSERT `noDup` SELECT DISTINCT * FROM `Dup` ;
DROP TABLE `Dup` ;
ALTER TABLE `noDup` RENAME `Dup` ;
Here is a simple answer:
delete a from target_table a left JOIN (select max(id_field) as id, field_being_repeated
from target_table GROUP BY field_being_repeated) b
on a.field_being_repeated = b.field_being_repeated
and a.id_field = b.id_field
where b.id_field is null;
This work for me to remove old records:
delete from table where id in
(select min(e.id)
from (select * from table) e
group by column1, column2
having count(*) > 1
);
You can replace min(e.id) to max(e.id) to remove newest records.
delete p from
product p
inner join (
select max(id) as id, url from product
group by url
having count(*) > 1
) unik on unik.url = p.url and unik.id != p.id;
I find Werner's solution above to be the most convenient because it works regardless of the presence of a primary key, doesn't mess with tables, uses future-proof plain sql, is very understandable.
As I stated in my comment, that solution hasn't been properly explained though.
So this is mine, based on it.
1) add a new boolean column
alter table mytable add tokeep boolean;
2) add a constraint on the duplicated columns AND the new column
alter table mytable add constraint preventdupe unique (mycol1, mycol2, tokeep);
3) set the boolean column to true. This will succeed only on one of the duplicated rows because of the new constraint
update ignore mytable set tokeep = true;
4) delete rows that have not been marked as tokeep
delete from mytable where tokeep is null;
5) drop the added column
alter table mytable drop tokeep;
I suggest that you keep the constraint you added, so that new duplicates are prevented in the future.
This procedure will remove all duplicates (incl multiples) in a table, keeping the last duplicate. This is an extension of Retrieving last record in each group
Hope this is useful to someone.
DROP TABLE IF EXISTS UniqueIDs;
CREATE Temporary table UniqueIDs (id Int(11));
INSERT INTO UniqueIDs
(SELECT T1.ID FROM Table T1 LEFT JOIN Table T2 ON
(T1.Field1 = T2.Field1 AND T1.Field2 = T2.Field2 #Comparison Fields
AND T1.ID < T2.ID)
WHERE T2.ID IS NULL);
DELETE FROM Table WHERE id NOT IN (SELECT ID FROM UniqueIDs);
Another easy way... using UPDATE IGNORE:
U have to use an index on one or more columns (type index).
Create a new temporary reference column (not part of the index). In this column, you mark the uniques in by updating it with ignore clause. Step by step:
Add a temporary reference column to mark the uniques:
ALTER TABLE `yourtable` ADD `unique` VARCHAR(3) NOT NULL AFTER `lastcolname`;
=> this will add a column to your table.
Update the table, try to mark everything as unique, but ignore possible errors due to to duplicate key issue (records will be skipped):
UPDATE IGNORE `yourtable` SET `unique` = 'Yes' WHERE 1;
=> you will find your duplicate records will not be marked as unique = 'Yes', in other words only one of each set of duplicate records will be marked as unique.
Delete everything that's not unique:
DELETE * FROM `yourtable` WHERE `unique` <> 'Yes';
=> This will remove all duplicate records.
Drop the column...
ALTER TABLE `yourtable` DROP `unique`;
If you want to keep the row with the lowest id value:
DELETE n1 FROM 'yourTableName' n1, 'yourTableName' n2 WHERE n1.id > n2.id AND n1.email = n2.email
If you want to keep the row with the highest id value:
DELETE n1 FROM 'yourTableName' n1, 'yourTableName' n2 WHERE n1.id < n2.id AND n1.email = n2.email
Deleting duplicates on MySQL tables is a common issue, that usually comes with specific needs. In case anyone is interested, here (Remove duplicate rows in MySQL) I explain how to use a temporary table to delete MySQL duplicates in a reliable and fast way, also valid to handle big data sources (with examples for different use cases).
Ali, in your case, you can run something like this:
-- create a new temporary table
CREATE TABLE tmp_table1 LIKE table1;
-- add a unique constraint
ALTER TABLE tmp_table1 ADD UNIQUE(sid, title);
-- scan over the table to insert entries
INSERT IGNORE INTO tmp_table1 SELECT * FROM table1 ORDER BY sid;
-- rename tables
RENAME TABLE table1 TO backup_table1, tmp_table1 TO table1;
delete from `table` where `table`.`SID` in
(
select t.SID from table t join table t1 on t.title = t1.title where t.SID > t1.SID
)
Love #eric's answer but it doesn't seem to work if you have a really big table (I'm getting The SELECT would examine more than MAX_JOIN_SIZE rows; check your WHERE and use SET SQL_BIG_SELECTS=1 or SET MAX_JOIN_SIZE=# if the SELECT is okay when I try to run it). So I limited the join query to only consider the duplicate rows and I ended up with:
DELETE a FROM penguins a
LEFT JOIN (SELECT COUNT(baz) AS num, MIN(baz) AS keepBaz, foo
FROM penguins
GROUP BY deviceId HAVING num > 1) b
ON a.baz != b.keepBaz
AND a.foo = b.foo
WHERE b.foo IS NOT NULL
The WHERE clause in this case allows MySQL to ignore any row that doesn't have a duplicate and will also ignore if this is the first instance of the duplicate so only subsequent duplicates will be ignored. Change MIN(baz) to MAX(baz) to keep the last instance instead of the first.
This works for large tables:
CREATE Temporary table duplicates AS select max(id) as id, url from links group by url having count(*) > 1;
DELETE l from links l inner join duplicates ld on ld.id = l.id WHERE ld.id IS NOT NULL;
To delete oldest change max(id) to min(id)
This here will make the column column_name into a primary key, and in the meantime ignore all errors. So it will delete the rows with a duplicate value for column_name.
ALTER IGNORE TABLE `table_name` ADD PRIMARY KEY (`column_name`);
I think this will work by basically copying the table and emptying it then putting only the distinct values back into it but please double check it before doing it on large amounts of data.
Creates a carbon copy of your table
create table temp_table like oldtablename;
insert temp_table select * from oldtablename;
Empties your original table
DELETE * from oldtablename;
Copies all distinct values from the copied table back to your original table
INSERT oldtablename SELECT * from temp_table group by firstname,lastname,dob
Deletes your temp table.
Drop Table temp_table
You need to group by aLL fields that you want to keep distinct.
DELETE T2
FROM table_name T1
JOIN same_table_name T2 ON (T1.title = T2.title AND T1.ID <> T2.ID)
here is how I usually eliminate duplicates
add a temporary column, name it whatever you want(i'll refer as active)
group by the fields that you think shouldn't be duplicate and set their active to 1, grouping by will select only one of duplicate values(will not select duplicates)for that columns
delete the ones with active zero
drop column active
optionally(if fits to your purposes), add unique index for those columns to not have duplicates again
You could just use a DISTINCT clause to select the "cleaned up" list (and here is a very easy example on how to do that).
Could it work if you count them, and then add a limit to your delete query leaving just one?
For example, if you have two or more, write your query like this:
DELETE FROM table WHERE SID = 1 LIMIT 1;
There are just a few basic steps when removing duplicate data from your table:
Back up your table!
Find the duplicate rows
Remove the duplicate rows
Here is the full tutorial: https://blog.teamsql.io/deleting-duplicate-data-3541485b3473