In SQL, how do update a table, setting a column to a different value for each row?
I want to update some rows in a PostgreSQL database, setting one column to a number from a sequence, where that column has a unique constraint. I hoped that I could just use:
update person set unique_number = (select nextval('number_sequence') );
but it seems that nextval is only called once, so the update uses the same number for every row, and I get a 'duplicate key violates unique constraint' error. What should I do instead?
Don't use a subselect, rather use the nextval function directly, like this:
update person set unique_number = nextval('number_sequence');
I consider pg's sequences a hack and signs that incremental integers aren't the best way to key rows. Although pgsql didn't get native support for UUIDs until 8.3
http://www.postgresql.org/docs/8.3/interactive/datatype-uuid.html
The benefits of UUID is that the combination are nearly infinite, unlike a random number which will hit a collision one day.
Related
I need to create a sequential number column for record number proposes
I am OK with losing sequence if I delete a row from the middle of the table
For example
1
2
3
If I delete 2, I am ok with new column been 4.
I tried to alter my table to
alter table [dbo].[mytable]
add [record_seq] as (MAX(record_seq) + 1)
but I am getting An aggregate may not appear in a computed column expression or check constraint.
Which is a bit confusing? do I need to specify an initial value? is there a better way?
If you're looking to allocate a sequence number even in cases where the table doesn't get a record inserted, I would handle it in the process responsible for performing those inserts. Create another table, in this table keep track of the max identity value of that sequence. Each time you want to perform an insert, reserve the sequence number you want by updating that table first. If you rely on selecting the max existing value, you could be at risk of multiple sessions getting the same "new" sequence number before inserting. Even if the insert fails, you will have incremented that control table so nothing else uses that value that has been reserved.
Its not supported in MsSql. You can use identity column:
ALTER TABLE [dbo].[mytable]
ADD [record_seq] INT IDENTITY
Or use trigger to update your seq column after insert and/or delete
I currently have a table with many rows, but no PK at all. I now require to have a unique, non-null, > 0 PK for every row.
I'm created the column for PK, but how I can I quickly fill in the column with an incremental value starting from 1?
Any method, a single SQL line, or a SQL line to be executed as many times as are rows are good enough for me.
Something like
update sometable set newkeyfield = Row_Number() Over();
should do not a DB2 bloke, but should be close.
Try using a DB2 Sequence object. They are designed for creating unique sequences of numbers. Pick your data type.
CREATE SEQUENCE mySeq as int;
You retrieve and increment the sequence in one step, using a sequence reference
NEXT VALUE FOR mySeq
You expression can use this in an INSERT, UPDATE, or MERGE, or most places you can use an expression.
I want to do something like this:
INSERT INTO T SELECT * FROM T WHERE Column1 = 'MagicValue' -- (multiple rows may be affected)
The problem is that T has a primary key column and so this causes an error as if trying to set the primary key. And frankly, I don't want to set the primary key either. I want to create entirely new rows with new primary keys but the rest of the fields being copied over from the original rows.
This is supposed to be generic code applicable to various tables. Well, so if there is no nice way of doing this, I will just write code to dynamically extract column names, construct the list etc. But maybe there is? Am I the first guy trying to create duplicate rows in a database or something?
I'm assuming by "Primary Key" you mean identity or guid data types that auto-assign or auto-increment.
Without some very fancy dynamic SQL, you can't do what you are after. If you want to insert everything but the identity field, you need to specify fields.
If you want to specify a value for that field, you need to specify all the fields in the SELECT and in the INSERT AND turn on IDENTITY_INSERT.
You don't gain anything from duplicating a row in a database (considering you didn't try to set the Primary Key). It would be wiser and will avoid problem to have another column called "amount" or something.
something like
UPDATE T SET Amount = Amount + 1 WHERE Column1 = 'MagicValue'
or if it can increase by more than 1 like amount of returned fields
Update T SET Amount = Amount * 2 WHERE Column1 = 'MagicValue'
I'm not sure what you're trying to do exactly but if the above doesn't work for what you're doing I think your design requires a new table and insert it there.
EDIT: Also as mentioned under your comments, a generic insert doesn't really make sense. Imagine, for this to work, you need the same number of fields, and they will hold the same values suggesting that they should also have the same names(even if it wouldn't require it to). It would basically be the same table structure twice.
I have a table, and there is no column which stores a field of when the record/row was added. How can I get the latest entry into this table? There would be two cases in this:
Loop through entire table and get the largest ID, if a numeric ID is being used as the identifier. But this would be very inefficient for a large table.
If a random string is being used as the identifier (which is probably very, very bad practise), then this would require more thinking (I personally have no idea other than my first point above).
If I have one field in each row of my table which is numeric, and I want to add it up to get a total (so row 1 has a field which is 3, row 2 has a field which is 7, I want to add all these up and return the total), how would this be done?
Thanks
1) If the id is incremental, "select max(id) as latest from mytable". If a random string was used, there should still be an incremental numeric primary key in addition. Add it. There is no reason not to have one, and databases are optimized to use such a primary key for relations.
2) "select sum(mynumfield) as total from mytable"
for the last thing use a SUM()
SELECT SUM(OrderPrice) AS OrderTotal FROM Orders
assuming they are all in the same column.
Your first question is a bit unclear, but if you want to know when a row was inserted (or updated), then the only way is to record the time when the insert/update occurs. Typically, you use a DEFAULT constraint for inserts and a trigger for updates.
If you want to know the maximum value (which may not necessarily be the last inserted row) then use MAX, as others have said:
SELECT MAX(SomeColumn) FROM dbo.SomeTable
If the column is indexed, MSSQL does not need to read the whole table to answer this query.
For the second question, just do this:
SELECT SUM(SomeColumn) FROM dbo.SomeTable
You might want to look into some SQL books and tutorials to pick up the basic syntax.
What would be the correct universal SQL construct to get the last row inserted (or it's primary key). The ID might be autogenerated by a sequence but I do not want to deal with the sequence at all! I need to get the ID by querying the table. Alternatively, INSERT might be somehow extended to return the ID. Assume I am always inserting a single row. The solution should work with most RDBMS!
the best way is to depend on the sequence like:
select Max(ID) from tableName
but If you don't want to deal with it, you can add new timestamp column to your table and then select max from that column.
like this way
select Max(TimestampField) from tableName