The results of an UPDATE statement are undefined if the statement includes a FROM clause that is not specified in such a way that only one value is available for each column occurrence that is updated, that is if the UPDATE statement is not deterministic. Modifying a text, ntext, or image column with UPDATE initializes the column, assigns a valid text pointer to it, and allocates at least one data page, unless the column is being updated with NULL.
For example, in the UPDATE statement in the following script, both rows in USE Adventure Works2012; GO IF OBJECT_ID ('dbo. To replace or modify large blocks of text, ntext, or image data, use WRITETEXT or UPDATETEXT instead of the UPDATE statement.
CURRENT OF Specifies that the update is performed at the current position of the specified cursor. Table1', ' U') IS NOT NULL DROP TABLE dbo. Table2', ' U') IS NOT NULL DROP TABLE dbo. Table1 (c1 int PRIMARY KEY NOT NULL, c2 int NOT NULL); GO CREATE TABLE dbo. Table2 WHERE CURRENT OF abc; GO SELECT c1, c2 FROM dbo.
A positioned update using a WHERE CURRENT OF clause updates the single row at the current position of the cursor. Table2 (d1 int PRIMARY KEY NOT NULL, d2 int NOT NULL); GO INSERT INTO dbo. Table2 VALUES (1, 20), (2, 30); GO DECLARE abc CURSOR LOCAL FOR SELECT c1, c2 FROM dbo. Table1; GO Support for use of the READUNCOMMITTED and NOLOCK hints in the FROM clause that apply to the target table of an UPDATE or DELETE statement will be removed in a future version of SQL Server.
This can be more accurate than a searched update that uses a WHERE ] ) Specifies that optimizer hints are used to customize the way the Database Engine processes the statement. Avoid using these hints in this context in new development work, and plan to modify applications that currently use them.
For more information, see Query Hints (Transact-SQL). All char and nchar columns are right-padded to the defined length.
Any characters not found in this code page are lost.
DEFAULT Specifies that the default value defined for the column is to replace the existing value in the column.Specifies the condition to be met for the rows to be updated.The search condition can also be the condition upon which a join is based.If the UPDATE statement affects multiple records, to return the old and new values for each record, use the OUTPUT clause. This can be configured in ODBC data sources or by setting connection attributes or properties.Use caution when specifying the FROM clause to provide the criteria for the update operation. For more information, see SET ANSI_PADDING (Transact-SQL).Specifies the temporary named result set or view, also known as common table expression (CTE), defined within the scope of the UPDATE statement.