I will try to explain you in a simple way what I mean in the title. Let's say that I have a table called [Persons] and each record in it is a person, having as columns:
Name, Surname and HashId.
Now, in my code I am able to search in this table by using as criteria Name and Surname, like for example:
SELECT *
FROM [Persons]
where Name = '..' AND Surname = '..'
This will return ALL the records that match the searching criteria. The 3rd column is a computed column that depends on the Name and Surname. So if I search for example someone called "Alex Foo" and in the database there are 2 people with the same name and surname, their HashID will be the same. What I want now is that my query doesn't return those 2 records but only one.. the query has to be able to check whether or not the HashID's are the same and in that case return only 1 record for all those that satisfy this rule. How can my modify my SQL query to reach this purpose? Thanks in advance!
EDIT: I omitted an information that now comes clear with your answers. I must filter over the HashId for the following reason: If there are 2 records with the name "Alex" and "alex", their HashId will be the same according to my logic, which is the correct logic for my purposes (the whole code behind is more complex than the explanation I gave you). But a simple select over the name "alex" would return the 2 different records with respectively name "Alex" and "alex". That's why the HashId is important.
Can't you just group by the name,Surname,HashId?
Like this:
SELECT name,Surname,HashId
FROM [Persons]
where Name = '..' AND Surname = '..'
GROUP BY name,Surname,HashId
If all there columns are the same then it will return one row for this
Edit
As Panagiotis Kanavos pointed out this will be the same as doing a distinct on the columns too. Maybe this is cleaner. Like this:
SELECT DISTINCT name,Surname,HashId
FROM [Persons]
where Name = '..' AND Surname = '..'
I'm having trouble getting this query to work in mysql:
UPDATE proyects_c
SET director=(SELECT users.keyid
FROM users,proyects
WHERE users.username=proyects.director
);
Now the problem is that the subquery returns more than one row. Thing is, this is what I want. The number of rows that it returns is the same as the number of rows in proyects_c, hence what I expected this to do is update every row in the column director with the result of the query.
However I get the error:
ERROR 1242 (21000) at line 23: Subquery returns more than 1 row
Which makes sense, but I can't get it to do what I want. What am I doing wrong?
As a secondary question, how can I split this into two queries? For clarity's sake.
Maybe something like that :
update proyects_c p
inner join users u on
u.username = p.director
set p.director = u.keyid
You need to specify "Top 1" in the select in T-SQL, or "LIMIT 1" (if memory serves) in MySQL to only return one row.
Try (assuming T-SQL):
UPDATE proyects_c SET director=(SELECT TOP 1 users.keyid FROM users,proyects WHERE users.username=proyects.director);
I did something to bypass the problem. The idea was given to me by Oscar Pérez. What I did is added a column to proyects_c (admin_id). Once that was done, this is the query I used:
UPDATE proyects_c, users SET proyects_c.admin_id=users.keyid WHERE proyects_c.director=users.username;
This did what I wanted. After that I deleted the director column and renamed admin_id to director. What I wanted to do is change the type of director from a string to an int in and add the int value from user.keyid corresponding the name (string) that used to be in director. So this worked for me. Oscar made me realize that the ONLY relation between the row I wanted to modify and the row from the query result was the position. I made it so that the relation was an equal value in two different fields. Thanks for the help.
In a large user database with the following format and sample data, we are trying to identify duplicated people:
id first_name last_name email
---------------------------------------------------
1 chris baker
2 chris baker chris#gmail.com
3 chris baker chris#hotmail.com
4 chris baker crayzyguy#crazy.com
5 carl castle castle#npr.org
6 mike rotch fakeuser#sample.com
I am using the following query:
SELECT
GROUP_CONCAT(id) AS "ids",
CONCAT(UPPER(first_name), UPPER(last_name)) AS "name",
COUNT(*) AS "duplicate_count"
FROM
users
GROUP BY
name
HAVING
duplicate_count > 1
This works great; I get a list of duplicates with the id numbers of the involved rows.
We would re-assign any associated data tied to a duplicate to the actual person (set user_id = 2 where user_id = 3), then we delete the duplicating user row.
The trouble comes after we make this report the first time, as we clean up the list after manually verifying that they are indeed duplicates -- some ARE NOT duplicates. There are 2 Chris Bakers that are legitimate users.
We don't want to keep seeing Chris Baker in subsequent duplicate reports until the end of time, so I am looking for a way to flag that user id 1 and user id 4 are NOT duplicates of each other for future reports, but they could be duplicated by new users added later.
What I tried
I added a is_not_duplicate field to the user table, but then if a new duplicate "Chris Baker" gets added to the database, it will cause this situation to not show on the duplicate report; the is_not_duplicate improperly excludes one of the accounts. My HAVING statement would not meet the > 1 threshold until there are -two- duplicates of Chris Baker, plus the "real" one marked is_not_duplicate.
Question Summed Up
How can I build exceptions into the above query without looping results or multiple queries?
Sub-queries are fine, but the size of the dataset makes every query count and I'd like the solution to be as performant as possible.
Try to add the is_not_duplicate boolean field and modify your code as follows:
SELECT
GROUP_CONCAT(id) AS "ids",
CONCAT(UPPER(first_name), UPPER(last_name)) AS "name",
COUNT(*) AS "duplicate_count",
SUM(is_not_duplicate) AS "real_count"
FROM
users
GROUP BY
name
HAVING
duplicate_count > 1
AND
duplicate_count - real_count > 0
Newly added duplicates will have is_not_duplicate=0 so the real_count for that name will be less than duplicate_count and the row will be shown
My brain is too fried to come up with the actual query for this at the moment, but I might be able to give you a nudge in a path that should work :)
What if you did add another column (maybe a table of valid duplicated users instead?...both will accomplish the same thing), and ran a subquery that would count up all of the valid duplicates and then you could compare against the count in your current query. You would exclude any users that have matching counts, and would pull in any with counts that are higher. Hopefully that makes sense; I will create a use case:
Chris Baker with id 1 and 4 are marked as valid_duplicates
There are 4 Chris Baker's in the system
You get a count of valid Chris Baker's
You get a count of all Chris Baker's
valid_count <> total_count, so return Chris Baker
*You probably can even modify the query so that it does not even list the duplicate id's (even if you get a duplicate marking of only 1 id). Rather than having to re-check which are the valids. This would be a little more complicated. Without it, at least you ignore Chris Baker until another enters the system
I have written up the basic query, dealing with excluding specific id's I will try to roll in tonight. But, this at least solves your initial need. If you do not need the more complicated query, do let me know so that I do not waste my time on it :)
SELECT
GROUP_CONCAT(id) AS "ids",
CONCAT(UPPER(first_name), UPPER(last_name)) AS "name",
COUNT(*) AS "duplicate_count"
FROM
users
WHERE NOT EXISTS
(
SELECT 1
FROM
(
SELECT
CONCAT(UPPER(first_name), UPPER(last_name)) AS "name",
COUNT(*) AS "valid_duplicate_count"
FROM
users
WHERE
is_valid_duplicate = 1 --true
GROUP BY
name
HAVING
valid_duplicate_count > 1
) AS duplicate_users
WHERE
duplicate_users.name = users.name
AND valid_duplicate_count = duplicate_count
)
GROUP BY
name
HAVING
duplicate_count > 1
Below is the query that should do the same as above, but the final list will only print the id's that are not in the valid list. This actually ended up being a lot simpler than I thought. And, it is mostly the same as above, but the only reason I kept above is to keep the two options and in case I messed the above up...it does get complicated as it is many nested queries. If CTE's are available to you, or even temp tables. It might make the query more expressive to break it up into temp tables :). Hopefully this helps and is what you are looking for
SELECT GROUP_CONCAT(id) AS "ids",
CONCAT(UPPER(first_name), UPPER(last_name)) AS "name",
COUNT(*) AS "final_duplicate_count"
--This count could actually be 1 due to the nature of the query
FROM
users
--get the list of duplicated user names
WHERE EXISTS
(
SELECT
CONCAT(UPPER(first_name), UPPER(last_name)) AS "name",
COUNT(*) AS "total_duplicate_count"
FROM
users AS total_dup_users
--ignore valid_users whose count still matches
WHERE NOT EXISTS
(
SELECT 1
FROM
(
SELECT
CONCAT(UPPER(first_name), UPPER(last_name)) AS "name",
COUNT(*) AS "valid_duplicate_count"
FROM
users AS valid_users
WHERE
is_valid_duplicate = 1 --true
GROUP BY
name
HAVING
valid_duplicate_count > 1
) AS duplicate_users
WHERE
--join inner table to outer table
duplicate_users.name = total_dup_users.name
--valid count check
AND valid_duplicate_count = total_duplicate_count
)
--join inner table to outer table
AND total_dup_users.Name = users.Name
GROUP BY
name
HAVING
duplicate_count > 1
)
--ignore users that are valid when doing the actual counts
AND NOT EXISTS
(
SELECT 1
FROM users AS valid
WHERE
--join inner table to outer table
users.name =
CONCAT(UPPER(valid.first_name), UPPER(valid.last_name))
--only valid users
AND valid.is_valid_duplicate = 1 --true
)
GROUP BY
FinalDuplicates.Name
Since this is basically a many-to-many relationship I would add a new table not_duplicate with fields user1 and user2.
I would probably add two rows for each not_duplicate relationship such that I have one row for 2 -> 3 and a symmetric row for 3 -> 2 to ease querying, but that may introduce data inconsistencies so make sure you delete both rows at the same time (or have only one row and make the correct query in your script).
well it seems to me that the is_not_duplicate column is not complex enough to hold the information you want to store - from what I understand you want to manually tell your detection that two distinct users are not duplicates of each other. so either you create a column like is_not_duplicate_of=other-user-id or if you want to keep the possibility open that one user can be manually defined not duplicate of more than one users, you need a seperate table with two user-id columns.
the query telling you the non overridden duplicates probably has to be a bit more complex than the one you suggested, I cannot think of one that works with a group by and having logic. The only thing that would come to my mind is something like
SELECT u1.* FROM users u1
INNER JOIN users u2
ON u1.id <> u2.id
AND u2.name = u1.name
WHERE NOT EXISTS (
SELECT *
FROM users_non_dups un
WHERE (un.id1 = u1.id AND un.id2 = u2.id)
OR (un.id1 = u2.id AND un.id2 = u1.id)
)
If you were to correct all duplicates each time you run the report, then a very simple solution might be to modify the query:
SELECT
GROUP_CONCAT(id) AS "ids",
MAX(id) AS "max_id",
CONCAT(UPPER(first_name), UPPER(last_name)) AS "name",
COUNT(*) AS "duplicate_count"
FROM
users
GROUP BY
name
HAVING
duplicate_count > 1
AND
max_id > MAX_ID_LAST_TIME_DUPLICATE_REPORT_WAS_GENERATED;
I would go ahead and make the "confirmed_unique" column, defaulted as "False."
In order to avoid the problems you mentioned,
Then I would select all elements that may look like duplicates and have a "False" entry for "confirmed_unique."
I am not sure if this will work, but could you consider the reverse logic of adding a *is_duplicate_of* column? That way you can mark duplicates by entering the ID of the first record at this column which will be greater than zero. The records that you wish to retain will have a 0 value at this field. You can set the default (unchecked records) to -1 to keep track of the validation status for each record.
Afterwards you can keep executing an SQL that will compare new records only with correct records having is_duplicate_of = 0 .
If you are ok to make a slight change to the format of the report. You could do a self-join like this -
SELECT
CONCAT(u1.id,",", u2.id) AS "ids",
CONCAT(UPPER(u1.first_name), UPPER(u1.last_name)) AS "name"
FROM
users u1, users u2
WHERE
u1.id < u2.id AND
UPPER(u1.first_name) = UPPER(u2.first_name) AND
UPPER(u1.last_name) = UPPER(u2.last_name) AND
CONCAT(u1.id,",", u2.id) NOT IN (SELECT ids from not_dupe)
which reports duplicates as follows:
ids | name
----|--------
1,2 | CHRISBAKER
1,3 | CHRISBAKER
...
And the not_dupe table would have rows like below:
ids
------
1,2
3,4
...
I think it would make sense to create a lookup-table storing the ids of the ones that are not duplicates. Thus confirmed non duplicants are removed and the query will only have to ad a small look up for duplicates actualy found on the lookup table.
for instance in this example we would have
id 1 | id 2
2 4
if crayzyguy#crazy.com and chris#gmail.com are diffrent persons.
If I were you, I will add some geolocalisation tables/fields to my database schema.
The probability two end-users are having the same names AND are living in the same place is very very low - except in very big town - but you can split geolocalization to small areas too - it's about granularity.
Good luck.
I would suggest you to create a couple of things:
A Boolean column to flag confirmed users
A String column to save ids
A trigger that will check if the first name and last name are already there to fill up the flag, and save in the string column all ids to which this one is a possible duplicate.
And then build a report that looks for duplicated true and decode the string field to match the possible duplicated
I gave Justin Pihony +1 as the 1st to suggest comparing the duplicate count with the not duplicate count, and Hrant Khachatrian +1 for being the 1st to show an efficient way of doing that.
Here is a slightly different method, plus some renaming to make everything a bit more self explanatory, plus some extra columns in the query to make it obvious which records need to be compared as potential duplicates.
I would call the new column "CONFIRMED_UNIQUE" instead of "IS_NOT_DUPLICATE". Like Hrant I would make it Boolean (tinyint(1) with 0=FALSE and 1=TRUE).
The "potential_duplicate_count" is the maximum number of records that would have to be deleted.
select
group_concat(case when not confirmed_unique then id end) as potential_duplicate_ids,
group_concat(case when confirmed_unique then id end) as confirmed_unique_ids,
concat(upper(first_name), upper(last_name)) as name,
sum( case when not confirmed_unique then 1 end ) - (not max(confirmed_unique)) as potential_duplicate_count
from
users
group by
name
having
potential_duplicate_count > 0
I see someone else has been voted down for the suggestion of merging, but nothing about your problem statement says the data needs to be inplace. The OP followed up with their solution which happens to be a put SQL one, that doesn't imply that every solution needs to be limited to that.
The issue as I understand is around contacts having multiple, similar, but not necessarily identical records in your database, which has cost and reputational implications so you're looking to deduplicate these records.
I would write a batch job that searches for potential duplicates (this can be as complicated or as simple as you like) and then close the two records that it finds are dupes and create a new record.
To enable that you'd need four new columns:
Status, which would be either Open, Merged, Split
RelatedId, which would hold the value of who the record was merged with
ChainId, the new record Id
DateStatusChanged, obvious enough
Open would be the default status
Merged would be when the record is merged (effectively closed and replaced)
Split would be if the merge was reversed
So, as an example, go through all of the records that, for example, have the same name. Merge them in pairs. So if you have three Chris Bakers, records 1, 2 and 3, merge 1 and 2 to make record 4 and then 3 and 4 to make record 5. Your table would end up something like:
ID NAME STATUS RELATEDID CHAINID DATESTATUSCHANGED [other rows omitted]
1 Chris Baker MERGED 2 4 27-AUG-2012
2 Chris Baker MERGED 1 4 27-AUG-2012
3 Chris Baker MERGED 4 5 28-AUG-2012
4 Chris Baker MERGED 3 5 28-AUG-2012
5 Chris Baker OPEN
This way you have a full record of what has happened to your data can reverse any changes by unmerging, if for example contacts 1 and 2 weren't the same you reverse the merge of 3 and 4, reverse the merge of 1 and 2, you'd end up with this:
ID NAME STATUS RELATEDID CHAINID DATESTATUSCHANGED
1 Chris Baker SPLIT 2 4 29-AUG-2012
2 Chris Baker SPLIT 1 4 29-AUG-2012
3 Chris Baker SPLIT 4 5 29-AUG-2012
4 Chris Baker CLOSED 3 5 29-AUG-2012
5 Chris Baker CLOSED 29-AUG-2012
You could then manually merge, as you'd probably not want your job to automatically remerge split records.
Is there a good reason for not merging duplicate accounts into a single account?
From the comments, it seems like the information is being used mostly for contact information so merging should be relatively painless and low risk. Once you merge users they will no longer appear in your duplicate report. Furthermore, you users table will actually shrink which could help with performance.
Add is_not_duplicate by datatype bit to your table and use below query after set is_not_duplicate data value:
SELECT GROUP_CONCAT(id) AS "ids",
CONCAT(UPPER(first_name), UPPER(last_name)) AS "name"
FROM users
GROUP BY name
HAVING COUNT(*) > SUM(CAST(is_not_duplicate AS INT))
above query compare total duplicate rows by total valid duplicate rows.
Why don't you make the email column to be a unique identifier in this case, and after you cleanse your records once, you do not allow duplicates from there onwards?
So I am trying to query through a table, call it students
if the column "status" in Student is 'active', then select the "firstName" and "lastName" coloumns from the previous record. Each record has a primary key.
That is, if the "status" column of record is =active, then, i want to select the previos record.
The table is already populated with data like
id firstname lastname status email
9 Joe Peters inactive fp#gmail.com
10 kim Rol active rt#yh.com
11 Lance Ree inactive lll&hh.com
12 diana Jones active sams#yahoo.com
For this data,the query should return Joe peters and lance ree
How can i do this?
thanks
If you simply want to select name from the records where the student is active, the query is a simple
SELECT id, firstname, lastname
FROM students
WHERE status = "active"
I assume what you mean by previous row is the row you just determined was an active student. The query will check your table for active students and retrieve their names. Because names are very rarely unique, I included the ID column.
I also recommend setting the status column to a 0 or 1 integer.
If you really want to select the previous row, you can use
SELECT sj.id, sj.firstname, sj.lastname
FROM students AS s
LEFT JOIN students AS sj ON (sj.id = s.id - 1)
WHERE s.status = "active"
The point is in determining the relation between the rows you have and the rows you want to select. In this case, you want to select the row which has the ID 1 less than your current row. Since this constraint is very restrictive, it's not a good idea to select rows this way. You might want to redesign your database.
With previous record you probably mean the priviously inserted one. you can refer to that row using last_insert_id() - which is a mysql-function - that will return the last inserted auto-incremented value.
select * from T where id = last_insert_id();
I need help for this problem.
In MYSQL Table i have a field :
Field : artist_list
Values : 1,5,3,401
I need to find all records for artist uid 401
I do this
SELECT uid FROM tbl WHERE artist_list IN ('401');
I have all record where artist_list fields values are '401' only, but if i have 11,401 this query do not match.
Any idea ?
(I cant user LIKE method because if artist uid is 3 (match for 30, 33, 3333)...
Short Term Solution
Use the FIND_IN_SET function:
SELECT uid
FROM tbl
WHERE FIND_IN_SET('401', artist_list) > 0
Long Term Solution
Normalize your data - this appears to be a many-to-many relationship already involving two tables. The comma separated list needs to be turned into a table of it's own:
ARTIST_LIST
artist_id (primary key, foreign key to ARTIST)
uid (primary key, foreign key to TBL)
Your database organization is a problem; you need to normalize it. Rather than having one row with a comma-separated list of values, you should do one value per row:
uid artist
1 401
1 11
1 5
2 5
2 4
2 2
Then you can query:
SELECT uid
FROM table
WHERE artist = 401
You should also look into database normalization because what you have is just going to cause more and more problems in the future.
SELECT uid
FROM tbl
WHERE CONCAT(',', artist_list, ',') LIKE '%,401,%'
Although it would make more sense to normalise your data properly in the first place. Then your query would become trivial and have much better performance.