I have a column of type float that contains phone numbers - I'm aware that this is bad, so I want to convert the column from float to nvarchar(max), converting the data appropriately so as not to lose data.
The conversion can apparently be handled correctly using the STR function (suggested here), but I'm not sure how to go about changing the column type and performing the conversion without creating a temporary column. I don't want to use a temporary column because we are doing this automatically a bunch of times in future and don't want to encounter performance impact from page splits (suggested here)
In Postgres you can add a "USING" option to your ALTER COLUMN statement that specifies how to convert the existing data. I can't find anything like this for TSQL. Is there a way I can do this in place?
Postgres example:
...ALTER COLUMN <column> TYPE <type> USING <func>(<column>);
Rather than use a temporary column in your table, use a (temporary) column in a temporary table. In short:
Create temp table with PK of your table + column you want to change (in the correct data type, of course)
select data into temp table using your conversion method
Change data type in actual table
Update actual table from temp table values
If the table is large, I'd suggest doing this in batches. Of course, if the table isn't large, worrying about page splits is premature optimization since doing a complete rebuild of the table and its indexes after the conversion would be cheap. Another question is: why nvarchar(max)? The data is phone numbers. Last time I checked, phone numbers were fairly short (certainly less than the 2 Gb that nvarchar(max) can hold) and non-unicode. Do some domain modeling to figure out the appropriate data size and you'll thank me later. Lastly, why would you do this "automatically a bunch of times in future"? Why not have the correct data type and insert the right values?
In sqlSever:
CREATE TABLE dbo.Employee
(
EmployeeID INT IDENTITY (1,1) NOT NULL
,FirstName VARCHAR(50) NULL
,MiddleName VARCHAR(50) NULL
,LastName VARCHAR(50) NULL
,DateHired datetime NOT NULL
)
-- Change the datatype to support 100 characters and make NOT NULL
ALTER TABLE dbo.Employee
ALTER COLUMN FirstName VARCHAR(100) NOT NULL
-- Change datatype and allow NULLs for DateHired
ALTER TABLE dbo.Employee
ALTER COLUMN DateHired SMALLDATETIME NULL
-- Set SPARSE columns for Middle Name (sql server 2008 only)
ALTER TABLE dbo.Employee
ALTER COLUMN MiddleName VARCHAR(100) SPARSE NULL
http://sqlserverplanet.com/ddl/alter-table-alter-column
Related
Is It possible to change varchar(10) column to data type numeric(10,4) using alter statement.
I tried it with alter command but getting an arror:
cannot convert column from varchar(10) to numeric(10,4)
You can't convert directly from VARCHAR to a numeric type, but you can work around the restriction by creating a temporary (numeric) column, populating it based on the current column, and then renaming it. When populating it, you'll need to make sure that the value is actually a number, for example that it does not contain commas as separators, punctuation like $, etc.
The documentation gives this example, where the original price column has values like '$50.00':
ALTER TABLE sales ADD COLUMN temp_price NUMERIC(10,2) DEFAULT
SUBSTR(sales.price, 2)::NUMERIC;
ALTER TABLE sales ALTER COLUMN temp_price DROP DEFAULT;
SELECT MAKE_AHM_NOW();
ALTER TABLE sales DROP COLUMN price CASCADE;
ALTER TABLE sales RENAME COLUMN temp_price to price;
I am creating a table in SQL Server for one of my customers that is needing an Access DB converted to SQL Server. ON the tables Access has masked a column a certain way. They have a number lets say TP001111 and when I pull the value into SQL Server I get 1111. I have a table called VendorNumber and I need to format this number.
CREATE TABLE VendorNumber
(
V_ID NVARCHAR(8) NOT NULL PRIMARY KEY,
V_Name NVARCHAR(30) NOT NULL
)
I have tried researching how to do this and I can't find a good example. What is the best way to have this value formatted when it is inserted into the table. I need to have it inserted as TP001111. The current code I have to format the column is:
UPDATE VendorNumber
SET V_ID = 'TP' + RIGHT((REPLICATE('0',6) + TPPRM_ID),6)
I have a table that was imported from another source as a column with the char datatype, but the field has international characters. I would like to use nvarcvhar in my table. How can I achieve this?
I already tried updating the columns (using an alter table statement, or "cast as") but the info stored did not get converted.
Thanks
like this
ALTER TABLE TableName ALTER COLUMN ColumnName NVARCHAR(size)
example
CREATE TABLE test (bla VARCHAR(50))
GO
ALTER TABLE test ALTER COLUMN bla NVARCHAR(50)
Make sure that you prefix the string with N when doing the insert
INSERT test VALUES (N'漢語')
SELECT * FROM test
output
bla
--------------------------------------------------
漢語
(1 row(s) affected)
Whatever you do, don't use the SSMS designer/wizard, it will recreate the whole table, see here for example When changing column data types use ALTER TABLE TableName ALTER Column syntax, don't drop and recreate column
alter table your_table alter column your_column nvarchar(length)
SQLFiddle example
Your data is currently in EUC-CN, masquerading as CP1252. It is not lost.
You have several approaches available for conversion to Unicode (look at Converting SQL databases section for overview). In case of SQL Server, you can create extended stored procedures for conversion from EUC-CN to Unicode. This will work but it is not exactly easy (in this case use code page 51936 for your data).
With some luck, depending on what particular characters occur in your data, and what language packs you have installed, you might be able to convert as if from code page 936 like this:
ALTER DATABASE mydatabasename COLLATE Chinese_PRC
If this succeeds, do the same for every column you are going to convert:
ALTER TABLE mytablename ALTER COLUMN mycolumnname
varchar(4000) COLLATE Chinese_PRC NOT NULL
And only then convert them to NVARCHAR.
I have couple tables with millions, and in some table billions, of rows, with one column as int now I am changing to bigint. I tried changing datatype using SSMS and it failed after a couple of hours as transaction log full.
Another approach I took is to create a new column and started updating value from old column to new column in batches, by setting ROWCOUNT property to 100000, it works but it very slow and it claims full server memory. With this approach, it may take a couple of days to complete, and it won't be acceptable in production.
What is the fast\best way to change datatype? The source column is not identity column and duplicate, and null is allowed. The table has an index on other columns, shall disabling index will speed up the process? Will adding Begin Tran and Commit help?
I ran a test for the ALTER COLUMN that shows the actual time required to make the change. The results show that the ALTER COLUMN is not instantaneous, and the time required grows linearly.
RecordCt Elapsed Mcs
----------- -----------
10000 184019
100000 1814181
1000000 18410841
My recommendation would be to batch it as you suggested. Create a new column, and pre-populate the column over time using a combination of ROWCOUNT and WAITFOR.
Code your script so that the WAITFOR value is read from a table. That way you can modify the WAITFOR value on-the-fly as your production server starts to bog down. You can shorten the WAITFOR during off-peak hours. (You can even use DMVs to make your WAITFOR value automatic, but this is certainly more complex.)
This is a complex update that will require planning and a lot of babysitting.
Rob
Here is the ALTER COLUMN test code.
USE tempdb;
SET NOCOUNT ON;
GO
IF EXISTS (SELECT * FROM sys.tables WHERE [object_id] = OBJECT_ID('dbo.TestTable'))
DROP TABLE dbo.TestTable;
GO
CREATE TABLE dbo.TestTable (
ColID int IDENTITY,
ColTest int NULL,
ColGuid uniqueidentifier DEFAULT NEWSEQUENTIALID()
);
GO
INSERT INTO dbo.TestTable DEFAULT VALUES;
GO 10000
UPDATE dbo.TestTable SET ColTest = ColID;
GO
DECLARE #t1 time(7) = SYSDATETIME();
DECLARE #t2 time(7);
ALTER TABLE dbo.TestTable ALTER COLUMN ColTest bigint NULL;
SET #t2 = SYSDATETIME();
SELECT
MAX(ColID) AS RecordCt,
DATEDIFF(mcs, #t1, #t2) AS [Elapsed Mcs]
FROM dbo.TestTable;
a simple alter table <table> alter column <column> bigint null should take basically no time. there won't be any conversion issues or null checks - i don't see why this wouldn't be relatively instant
if you do it through the GUI, it'll probably try to create a temp table, drop the existing table, and create a new one - definitely don't do that
In SQL Server 2016+, this alter table <table> alter column <column> bigint null statement will be a simple metadata change (instant) if the table is fully compressed.
More info here from #Paul White:
https://sqlperformance.com/2020/04/database-design/new-metadata-column-changes-sql-server-2016
Compression must be enabled:
On all indexes and partitions, including the base heap or clustered index.
Either ROW or PAGE compression.
Indexes and partitions may use a mixture of these compression levels. The important thing is there are no uncompressed indexes or partitions.
Changing from NULL to NOT NULL is not allowed.
The following integer type changes are supported:
smallint to integer or bigint.
integer to bigint.
smallmoney to money (uses integer representation internally).
The following string and binary type changes are supported:
char(n) to char(m) or varchar(m)
nchar(n) to nchar(m) or nvarchar(m)
binary(n) to binary(m) or varbinary(m)
All of the above only for n < m and m != max
Collation changes are not allowed
I have a SQL Server 2008 table which contains an external user reference currently stored as a bigint - the userid from the external table. I want to extend this to allow email address, open ID etc to be used as the external identifier. Is it possible to alter the column datatype from bigint to varchar without affecting any of the existing data?
Yes, that should be possible, no problem - as long as you make your VARCHAR field big enough to hold you BIGINT values :-)
You'd have to use something like this T-SQL:
ALTER TABLE dbo.YourTable
ALTER COLUMN YourColumnName VARCHAR(50) -- or whatever you want
and that should be it! Since all BIGINT values can be converted into a string, that command should work just fine and without any danger of losing data.