I'm using IBExpert to operate my databases. I have an old table with two fields:
- code (char 3) PK + INDEX
- name (varchar 30)
The new table should be
- code (integer) PK + INDEX
- name (varchar 30)
The data in the code column consist of only numbers even if it was a char type.
Is it possible to change the type of the code column? If yes how? I assume that I need to drop the primary key and delete or deactivate the index, but IBExpert won't let me do it and gives me this error when I try to drop the PK: object INDEX is in use
The error object INDEX is in use means exactly that: the object is in use, for example in the query plan of a prepared statement. To be able to modify it, you'll need to ensure nothing is holding an existence lock on that object, and the simplest way to do that is to shutdown the database using gfix, make your changes and set your database online again.
Related
I want Write ALTER TABLE SQL statement to add a column to the table. The column is classified as NUMBER datatype, NOT NULL attribute, and primary key.
But it shows ORA-01758.
ALTER TABLE INSURANCE
ADD (INS_ID NUMBER PRIMARY KEY NOT NULL);
If I select DEFAULT 0, it really solves the problem, but I cannot set up a primary key and INS_ID shows 0, not (null)
Because this table's data is from a excel document, what should I solve it without delete data?
If I must delete data how restore it easily?
Typically you can either:
provide a default value so oracle can fill the column as it creates, satisfying the constraint or
create the column as nullable, fill it with relevant data, then enable the not null restriction/make it the primary key after it has data or
empty the table
1 is not an option for you, because the values will have to be unique if they are to be a primary key. You could consider associating the column with a sequence or making it an identity column though
2 is a likely option for you if an auto generated incrementing number is no good as a PK (for example the key data is already known or calculated)
3 is something you've already said is not an option
Give some thought to the ongoing maintenance requirements - every front end app that writes data into this table will need to be upgraded to understand it has a primary key unless you're using a sequence/identity or similar that provides a unique value for the row. If there will be a lot to update and you dont care to have a PK in a particular form or from some existing value/relationship elsewhere, having an auto number PK can be helpful. If this data needs to relate to existing data that has a key, you need to upgrade front end apps so they can respect the new PK
is it possible to create a autoserial index in order 1,2,3,4... in Informix and what would be the syntax. I have a query and some of my timestamps are identical so I was unable to query using a timestamp variable. Thanks!
These are the commands that I ran to add an id field to an existing table. While logged in to the dbaccess console.
alter table my_table add id integer before some_field;
create sequence myseq;
update my_table set id = myseq.nextval;
drop sequence myseq;
alter table my_table modify (id serial not null);
Thanks to #ricardo-henriques for pointing me in the right direction. These commands will allow you to run the instructions explained in his answer on your database.
That would be the SERIAL data type.
You can use, as #RET mention the SERIAL data type.
Next you will struggle with the fact that you can't add a SERIAL column to an existing table. Ways to work around:
Add an INTEGER column, populate with sequential numbers and then alter the column to SERIAL.
Unload the data to a file, drop the table and recreate it with the new column.
Create a new table with the new column, populate the new table with the data from the old, drop the old and rename the new.
...
Bear in mind that they may not be unique. Hence you have to create an unique index or a primary key or an unique constraint in the column to prevent duplicates.
Another notes you should be aware:
- Primary key don't allow NULLS, unique index and unique constraints allow (as long there is only one record), so you should specify NOT NULL on the column definition.
- If you use a primary key or a unique constraint you can create a foreign key to it.
- In primary key and unique constraint the validation of the uniqueness of the record is done in the end of the DML, for the unique index it is done row a row.
Seems you're getting your first touch with informix, welcome. Yes it can be a little bit hard on the beginning just remember:
Always search before asking, really search.
When in doubt or reached a dead end then ask away.
Try to trim down your case scenario, built your own case the simple away you can, these will not only help us to help us but you will practice and in some cases find the solution by yourself.
When error is involve always give the error code, in informix it is given at least one error code and sometimes an ISAM error too.
Keen regards.
In my project I use a sqlite database, unfortunately my friend have make an error. A field is with no correct type.
So for the moment when the data in the field 'localid' (declare as integer) is more than 2147483647, all entry in this field is set to the max 2147483647.
The alter table/alter column sql request do not works with sqlite because it supports a limited subset of ALTER TABLE : only rename and add a new column.
So how can I make a change without data loss? create a new database correctly, coppy all data into it and delete the old?
But maybe there is a better way ? Someone have an idea?
Proceed like this:
create temporary table that contains fields that form the primary
key and localid (I assume this is nor PK).
fill temporary table
drop old column
add new column
fill new column by selecting from temporary table (+ possible conversion to new type).
Don't forget possible foreign keys if column is used as such and remember possibility to temporally relax constraint if it makes conversion smooth (likely not needed in your case).
Let's say I have a table defined as follows:
CREATE TABLE SomeTable
(
P_Id int PRIMARY KEY IDENTITY,
CompoundKey varchar(255) NOT NULL,
)
CompoundKey is a string with the primary key P_Id concatenated to the end, like Foo00000001 which comes from "Foo" + 00000001. At the moment, entries insertions into this table happen in 2 steps.
Insert a dummy record with a place holder string for CompoundKey.
Update the CompoundKey with the column with the generated compound key.
I'm looking for a way to avoid the 2nd update entirely and do it all with one insert statement. Is this possible? I'm using MS SQL Server 2005.
p.s. I agree that this is not the most sensible schema in the world, and this schema will be refactored (and properly normalized) but I'm unable to make changes to the schema for now.
Your could use a computed column; change the schema to read:
CREATE TABLE SomeTable
(
P_Id int PRIMARY KEY IDENTITY,
CompoundKeyPrefix varchar(255) NOT NULL,
CompoundKey AS CompoundKeyPrefix + CAST(P_Id AS VARCHAR(10))
)
This way, SQL Server will automagically give you your compound key in a new column, and will automatically maintain it for you. You may also want to look into the PERSIST keyword for computed columns which will cause SQL Server to materialise the value in the data files rather than having to compute it on the fly. You can also add an index against the column should you so wish.
A trigger would easily accomplish this
This is simply not possible.
The "next ID" doesn't exist and thus cannot be read to fulfill the UPDATE until the row is inserted.
Now, if you were sourcing your autonumbers from somwhere else you could, but I don't think that's a good answer to your question.
Even if you want to use triggers, an UPDATE is still executed even if you don't manually execute it.
You can obscure the population of the CompoundKey, but at the end of the day it's still going to be an UPDATE
I think your safest bet is just to make sure the UPDATE is in the same transaction as the INSERT or use a trigger. But, for the academic argument of it, an UPDATE still occurs.
Two things:
1) if you end up using two inserts, you must use transaction! Otherwise other processes may see the database in inconsistent state (i.e. seeing record without CompoundKey).
2) I would refrain from trying to paste the Id to the end of CompoundKey in transaction, trigger etc. It is much cleaner to do it at the output if you need it, e.g. in queries (select concat(CompoundKey, Id) as CompoundKeyId ...). If you need it as a foreign key in other tables, just use the pair (CompoundKey, Id).
How to create a unique constraint on a varchar(max) field in visual studio, visually.
the problem is when i try it:
manage indexes and keys > add > columns
I can only chose the bigint columns, but not any of the varchar(max) ones.
Do I maybe have to use check constraints?
If yes, what to put in the expression?
Thnx for the info
You cannot put a unique constraint on a VARCHAR(MAX) column (which could be up to 2 GB of text!!). You just simply cannot.
The unique constraint is enforced by a unique index in the background, and SQL Server has a 900 byte limit on index entries. You also cannot put a unique constraint on a VARCHAR(2000) field for that reason.
You'll need to find another way to achieve what you're trying to do. You could e.g. calculate the length and something like a checksum over your text and put a unique constraint on those length and checksum columns.
One way to do this would be to add a column for a hash that is calculated whenever you insert or update the column and put a unique index on that. While hash collisions do happen, it is extremely unlikely.
You could use this T-SQL keyword:
http://msdn.microsoft.com/en-us/library/ms174415.aspx
Even if this were possible, it would be a bad idea.
1) There is another way. Find some other data to use as your unique column
2) If you ABSOLUTELY HAVE TO use the varchar(Max). Maybe hash it on insert/update and add a hash column?