We have a giant table in an Access database with over 500k records and no PK. Is it possible to insert an auto-incrementing primary key column into an already existing Access table?
Yes, it is and can be done quite simply by editing the table and adding an autoincrement type field. The only rule is that you can only have one autoincrement per table.
Related
The table already has duplicates entries. I want to create a unique constraint in MQSL DB without deleting the existing duplicates. If any duplicate entries coming onwards then it will show an error. Given blow queries not working in MYSQL.
ALTER TABLE presence
ADD CONSTRAINT present uniqueness UNIQUE (employee_id,roll_number) where id >10000;
or
ALTER TABLE presence
ADD CONSTRAINT present uniqueness UNIQUE (employee_id,roll_number) where id <> (343,34534,34534)
Do we have something like that solution in SQL?
Add an additional column to the table that indicates the existing values.
Set it to NULL for the existing values. And give it a constant value, say 1, for the new rows. Then create a unique index or constraint on this column:
alter table t add constraint unique (employee_id, is_old)
Actually, I realize that you probably don't want duplicates with singleton old values and new values. That is just an issue of setting the value to NULL only for duplicates in the history. So, one row would have a constant value (say 1) in the historical data.
MySQL allows duplicate values on NULL, which is why this works.
I am creating a MySQL table with a primary key column; should I also select the auto-increment option or does primary key already do that?
Definitely You should select both.
and the reason why you select both is below
Auto-increment allows a unique number to be generated automatically
when a new record is inserted into a table.
Often this is the primary key field that we would like to be created
automatically every time a new record is inserted.
when creating a new table i'm defining a field called ID with index primary and autoincrement.
when adding new records it's not possible defining the same ID twice so i was wondering: what's the deal about setting index "unique" for ID? is it required, will it improve performance? thanks
You dont have to set it unique, by setting it as auto increment and primary key you have already forced uniqueness over there. You dont need seperate unique constraint here.
You can not add the multiple records with the same ID.
if you want to create the multiple record , firstly delete the primary or auto incr...
Then it will be inserted.
I have a bunch of huge tables that don't have primary keys. (Don't ask me why) I will append an 'id' field to each table. It will be an integer type. Later, I will promote it to a non-null, unique-value index, and a primary key.
My question: Is there a way in MySQL (5 ish) We have about a hundred tables, and the largest among them have over 1 million records. After creating the new 'id' column, is there a way to have MySQL backfill (ie, add a value to the existing records) the 'id' field? I would rather be able to do this all in MySQL. Otherwise I will have to write a PHP script to populate the existing records.
Thanx, Don!
If you do a
ALTER TABLE table1 ADD COLUMN id INTEGER NOT NULL auto_increment PRIMARY KEY
It will auto populate your table with a auto_incrementing primary key.
Might take a while on a large table.
I have some records that I need to store in a database table and I want to distinguish one record from other records on the basis of name field.
But as the datatype of name field is varchar, it will effect the performance because comparing varchar data with each other is time consuming process in comparision to a numeric field.
So I want each record to have a unique numeric field (say id). But if I make the id as primary key, then the more than one record can contain same name.
What's the solution to the above problem?
You can still create a UNIQUE constraint on the name field:
ALTER TABLE your_table ADD UNIQUE (name);
The UNIQUE constraint, like the PRIMARY KEY constraint, will guarantee that your column will contain only unique values.
Note that you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint.
Name can be made unique index.