Is it possible to define a default value using a where clause:
Name | Value
------| ------
Jeff | Green
Paul | Red
Seb | Blue
ALTER TABLE Blokes
MODIFY Name DEFAULT Green WHERE Name='Jeff'
Something along these lines.
(apologies for the poor formatting but I couldn't get the table to work)
Edit: I am looking to update future entries automatically using DEFAULT. The DB in use is Oracle.
Edit 2: I was unable to find what I needed so I have gone with the option to update the data in a php script.
You can add a computed column:
ALTER TABLE
Blokes
ADD Name AS (CASE WHEN Name ='Jeff' THEN 'N' ELSE Name END)
Related
I am using MS SQL Server 2012. I have this bit of SQL:
alter table SomeTable
add Active bit not null default 1
In some environments the default value is applied to existing rows and in other environments we have to add an update script to set the new field to 1. Naturally I am thinking that the difference is a SQL Server setting but my searches thus far are not suggesting which one. Any suggestions?
Let me know if the values of particular settings are desired.
Edit: In the environments that don't apply the default the existing rows are set to 0, which at least conforms to the NOT NULL.
If you add the column as not null it will be set to the default value for existing rows.
If you add the column as null it will be null despite having a default constraint when added to the table.
For example:
create table SomeTable (id int);
insert into SomeTable values (1);
alter table SomeTable add Active_NotNull bit not null default 1;
alter table SomeTable add Active_Null bit null default 1;
select * from SomeTable;
returns:
+----+----------------+-------------+
| id | Active_NotNull | Active_Null |
+----+----------------+-------------+
| 1 | 1 | NULL |
+----+----------------+-------------+
dbfiddle.uk demo: http://dbfiddle.uk/?rdbms=sqlserver_2016&fiddle=c4aeea808684de48097ff44d391c9954
Default value will be applied to existing row to avoid violation of "NOT NULL" constraint.
When using multiple SETs on a single update query like
update table set col1=value1,col2=col1
is there an order of execution that will decide the outcome, when the same column is left or right of an equals sign? As far as I've tested so far, it seems when a column is used to the right of an equals as a data source, then its value is used from BEFORE it gets a new value within the same update statement, by being to the left of an equals sign elsewhere.
I believe that SQL Server always uses the old values when performing an UPDATE. This would best be explained by showing some sample data for your table:
col1 | col2
1 | 3
2 | 8
3 | 10
update table set col1=value1,col2=col1
At the end of this UPDATE, the table should look like this:
col1 | col2
value1 | 1
value1 | 2
value1 | 3
This behavior for UPDATE is part of the ANSI-92 SQL standard, as this SO question discusses:
SQL UPDATE read column values before setting
Here is another link which discusses this problem with an example:
http://dba.fyicenter.com/faq/sql_server/Using_Old_Values_to_Define_New_Values_in_UPDATE_Statements.html
You can assume that in general SQL Server puts some sort of lock on the table during an UPDATE, and uses a snapshot of the old values throughout the entire UPDATE statement.
I'm wondering if it's possible to have a column always kept in sync with another column in the same table.
Let this table be an example:
+------+-----------+
| name | name_copy |
+------+-----------+
| John | John |
+------+-----------+
| Mary | Mary |
+------+-----------+
I'd like to:
Be able to INSERT into this table, using providing a value only for the name column - The name_copy column should automatically take the value I used in name
When UPDATE-ing the name column on a pre-existing row, the name_copy should automatically update to match the new & updated name_column.
Some solutions
I could do this via code but that would be terribly bad as there's no guarantee the data would always be accessible by my code (what if someone changes the data through a DB client?)
What would be a safe and reliable and easy way to tackle this in Postgres?
You can create a trigger. Simple trigger function:
create or replace function trigger_on_example()
returns trigger language plpgsql as $$
begin
new.name_copy := new.name;
return new;
end
$$;
In Postgres 12+ there is a nice alternative in the form of generated columns.
create table my_table(
id int,
name text,
name_copy text generated always as (name) stored);
Note that a generated column cannot be written to directly.
Test both solutions in db<>fiddle.
Don't put name_copy into the table. One method is to create the column and access it using a view:
create view v_table as
select t.*, name as name_copy
from t;
That said, I don't really see a use for this.
If I have the following table:
| name | value |
------------------
| A | 1 |
| B | NULL |
Where at the moment name is of type varchar(10) and value is of type bit.
I want to change this table so that value is actually a nvarchar(3) however, and I don't want to lose any of the information during the change. So in the end I want to end up with a table that looks like this:
| name | value |
------------------
| A | Yes |
| B | No |
What is the best way to convert this column from one type to another, and also convert all of the data in it according to a pre-determined translation?
NOTE: I am aware that if I was converting, say, a varchar(50) to varchar(200), or an int to a bigint, then I can just alter the table. But I require a similar procedure for a bit to a nvarchar, which will not work in this manner.
The best option is to ALTER bit to varchar and then run an update to change 1 to 'Yes' and 0 or NULL to 'No'
This way you don't have to create a new column and then rename it later.
Alex K's comment to my question was the best.
Simplest and safest; Add a new column, update with transform, drop existing column, rename new column
Transforming each item with a simple:
UPDATE Table
SET temp_col = CASE
WHEN value=1
THEN 'yes'
ELSE 'no'
END
You should be able to change the data type from a bit to an nvarchar(3) without issue. The values will just turn from a bit 1 to a string "1". After that you can run some SQL to update the "1" to "Yes" and "0" to "No".
I don't have SQL Server 2008 locally, but did try on 2012. Create a small table and test before trying and create a backup of your data to be safe.
i wanted to change EN_NO length from 21 to 16 in sql table TB_TRANSACTION. Below are mine current sql column fields.
sql command -
describe table tb_transaction
column | type schema | type name | length | scale | nulls
EN_NO| SYSIBM | VARCHAR | 21 | 0 | Yes
i tried with this command but failed.
alter table tb_transaction alter column EN_NO set data type varchar(16)<br/>
Error message:
SQL0190N ALTER TABLE "EASC.TB_TRANSACTION" specified attributes for column
"EN_NO" that are not compatible with the existing column. SQLSTATE=42837
Any help would be appreciated.
We can increase the size of the column but, we can not decrease the size of column because Data lose will be happen that's why system will not allow to decrease the size.
If you still want to decrease the size, you need to drop that column and add again.