I've created a table schema and specified that for some attributes, values cannot be null. For one column of this table, values are to be imported from a column of some another table but the problem i am facing is that when i use insert statement to copy values from that column of another table to the column of this newly created table, the attributes of this new column start screaming because they kind of have a constraint on them that while insertion their values cannot be NULL!
How do i cope with this?
One solution is that for other attributes, just for time being, i can state that null values can be accommodated so that i can successfully import values from column of other table and then later on put condition on the remaining attributes that values are not be NULL. But how do i do do this?
You need to convert NULL to some DEFAULT values while importing.
I am not sure which DB engine you are using, in mysql:
Use something like IFNULL(column_name, "").
Reference
You may simply be looking for the default clause. When you define a column, you can specify;
intcol int not null default 0
If the column is not specified for an insert, then it will default to 0. In some databases, if a NULL value is supplied, it will also get the default value.
Related
I have a table Rates with data in it and I need to add new column to the table however I get the error:
ALTER TABLE only allows columns to be added that can contain nulls, or have a DEFAULT definition specified, or the column being added is an identity or timestamp column, or alternatively if none of the previous conditions are satisfied the table must be empty to allow addition of this column. Column 'CreatedOn' cannot be added to non-empty table 'RateIncreases' because it does not satisfy these conditions
How can I do this, I have disabled prevent saving changes that required table re-creation
What part of the error do you not understand? When a column is added to an existing table, there are rows. The column is assigned to each of those rows. SQL Server has to give those rows a value. How does it do this?
It can assign the default value for the column.
It can assign NULL.
In your case, you have defined the column as NOT NULL but not provided a default value. Hence, the database does not know what to do, so it returns an error.
The simplest solution is to remove the NOT NULL constraint in the definition. Very close behind is assigning a default value.
I have a sql table which data type is int and it doesn’t accept null value.
So what I want to do is add “null” using derived column.
What I did is used drived transformation and add a new column and use expression (DT_WSTR,10) “null”
And then used data conversion and changed the data type into DT_14 but the data conversion fails upon execution.
Is there any other way to do this?
You can't do what you're trying to do. An INTEGER NOT NULL column will throw an error if you try to insert a text value into it, as you've seen.
There are really only two options.
Insert a zero for any NULL values that come through.
Insert a dummy value that's out of the range of values for the column, such as 999999 or the minimum or maximum values for an integer data type.
Or, of course, as Gordon suggested in the comments, drop the NOT NULL constraint on the column and insert the NULL values.
if a table have default value on a column for e.g.
create table emp
(
flag smallint default 1
)
so is there any impact of this default column in bulk import , if I am not using in insert statement.
According to the docs:
Before 11:
Adding a column with a default requires updating each row of the table (to store the new column value). However, if no default is specified, PostgreSQL is able to avoid the physical update. So if you intend to fill the column with mostly nondefault values, it's best to add the column with no default, insert the correct values using UPDATE, and then add any desired default as described below.
source
11 and after:
From PostgreSQL 11, adding a column with a constant default value no longer means that each row of the table needs to be updated when the ALTER TABLE statement is executed. Instead, the default value will be returned the next time the row is accessed, and applied when the table is rewritten, making the ALTER TABLE very fast even on large tables. However, if the default value is volatile (e.g., clock_timestamp()) each row will need to be updated with the value calculated at the time ALTER TABLE is executed. To avoid a potentially lengthy update operation, particularly if you intend to fill the column with mostly nondefault values anyway, it may be preferable to add the column with no default, insert the correct values using UPDATE, and then add any desired default as described below.
source
If you take a look into the source for the generator/planner/optimizer/rewriter
( postgresql/src/backend/rewrite/rewriteHandler.c around line#1112, function build_column_default() ) :
The default value(or function, e.g. for serials) is fetched from the catalogs and added once to the query tree. So, the DEFAULT may even be more efficient than fetching separate values for all affected rows from one of the tables in query.
But you would have to measure the difference to be sure.
I am working in SQL Server 2008. I have a table with many columns that will not have values (at least, for the given situation). So, they will have a NULL value when I query each of them. I would like to instead make these NULL values be empty strings (i.e., ''). What is the best way to achieve this? My current idea is to set a DEFAULT value of '' on each them at the time that the table is created. However, since there are so many of them, this will be very tedious.
You have 2 options:
As you said, give it a default value of empty string for columns you don't want to be null when you create table/add new columns.
When you select nullable columns from the table you can use IsNull(ColumnName,'') which means if ColumnName is null it'll return empty string ('').
Create a table with the same structure as your current table, with a different name, and the default value as ''.
Insert into that table from your original table.
Delete the original table.
Change the name of the new table to the original table name.
I want to add or update columns using alter table if i am adding a new column i want show error. I am using the code below
alter table Personal_Details alter columns DOB datetime
if i uncheck the NULL to not NULL then it will shows column does not allow nulls; update fails;
i want to insert the fields in between columns not at end.
Plese fix my bug,
Thanks in advance.
The position of the column in the table declaration has nothing to do with its being NULL or NOT NULL.
If you are adding a column (of any type) which you want to be NOT NULL, i.e. you want to prohibit NULL values in that column, and the table already contains some rows, you must also provide some default value. For example:
ALTER TABLE Personal_Details
ADD COLUMN DOB datetime NOT NULL DEFAULT (GETDATE())
Otherwise the engine will attempt to add that column with NULLs as its values, which will violate the NOT NULL property, and the change, therefore, will be reverted.
Basically, the same applies when you want to set an existing column's NOT NULL property on while the column already contains NULLs. But in this case you must explicitly eliminate the NULLs before the change by either replacing them with values or removing the respective rows.
Source:
ALTER TABLE (Transact-SQL). (The particular section related to your problem is just above this code snippet.)
1)For ur adding column with not null problem
Use
ALTER TABLE Personal_Details ADD COLUMN DOB datetime NULL
Update the DOB column with the required dates and make sure there is no null in the column
then alter the column using
ALTER TABLE Personal_Details ALTER COLUMN DOB datetime not NULL
2)For your column going to the end problem...
you should not be worried...the order in which the columns are arranged doesnt matter...unless u are using a pathetic way of accessing data by column order..in which case again..u should stop accessing it by column order...
If the column order really matters you can change it using design option in the sql management table(rightclick on table >design and drag the column to its required place.)