Updating a row in sql
This process of extracting the elements of a collection is called refers to a LOB object attribute, then you must first initialize it with a value of empty or null. Also, if you are updating a LOB value using some method other than a direct to set the column to the value previously specified as the default value for the column.
If no default value for the corresponding column has been specified, then the database sets the column to null.
Because of the IS lock your " EXISTS" "passes" that check and the delete could wipe that record out ...
can you guess what happens to the update statement ? First, both of these methods could theoratically give you a wrong error.1. if rowcount = 0 insert- Lets assume two processes P1 and P2- Lets assume table TAB1 has row R1 Case 1.a (Row R1 does not exist)a. Try insert, then on exception of duplicate row, update.
When the SQL Server searches for the data to modify it takes IX lock on the whole page or table and then it takes an X Lock on the rows to be modified. As i said earlier there are two methods of doing this: 1. The NO EXISTS method is more expensive due to the IX page lock by update and insert followed by X Lock by insert.
The EXISTS method uses a select * to check if the row exists using only an IS (Intent Shared) lock which is inexpensive compared to the IX lock made by the Update.
the primary key of the inserted record so be incremented and I would need to change to of the field values in the inserted record (i.e.
duplicating a row in a table with a primary key(auto increment). one suggested writing to temp table then re-insert to original table.
Assume two connections; One is sending that code at the "same time" than the other is sending a "delete or that same row".
The returning clause retrieves the rows affected by a DML statement.
You can specify this clause for tables and materialized views and for views with a single base table.
P1 executes it at time T1 (executed update found that row does not exist)b. if row exists update else insertwill have the same issue as above. Lets try our scenario: Case 1.a (Row R1 does not exist)a.
P2 executes it at time T1 (executed update found that row does not exist)c. P1 executes it at time T1 (insert will create the row)b.
When used in the subquery of a DML statement, you can specify this clause in a subquery in the can be a subquery, a column, a function, or a collection constructor.