Here, we have the basic syntax describing the ALTER TABLE MODIFY statement and showing examples accordingly: ALTER TABLE TableName We will use the ALTER TABLE MODIFY command to make some changes in the structure of existing columns in the Products table. You can see a new structure of the table formed using the ALTER TABLE ADD COLUMN statements. Suppose we are adding two more columns in the Products table which is shown below with the help of ALTER query: ALTER TABLE Products Let us add multiple columns to the Products table using the succeeding syntax for ALTER TABLE ADD Query:ĪDD NewColumnNameCol_Definition, You can view the column lists of the Products table using the query below in the database:Īs you can see in the output above that a new column, Price, is now added to the Products table at the end of the lists. Now, we will apply the ALTER TABLE ADD COLUMN statement to add a new column at the end of the columns present in the Products table in the database: ALTER TABLE Products ADD Cost INT NOT NULL But if these options are not provided in the ALTER ADD COLUMN query, the new table column will be affixed at the end of the list of columns in the table. There are two options: add the table column after the last column existing in the table using AFTER ColumnNameKeyword or, as the first column using FIRST ColumnNamekeyword. FIRST | AFTER ColumnName: It determines the position of the new table column to be added.Col_Definition: It includes the Data Type for the new column added, maximum size, and valid CONSTRAINTS.NewColumnName: Mention the name of the new table column to be added.TableName: We will provide the specific table name after the keywords ALTER TABLE where we want to supplement a new column or columns.Here, the above terms are explained as follows: However, from SQL Server 2012 onwards, the approach has changed so that adding a column with default constraints is much better.Let us add a column to the Products table using the succeeding syntax for ALTER TABLE ADD Query.ĪDD NewColumnNameCol_Definition Therefore, in the prior SQL Server 2012 era, rather than adding a column with a default constraint, it is better to add a column and updating with batches so that the table is not exclusively locked. We covered the basic syntax of SQL Server ALTER TABLE in this article and implemented it to add columns to an existing table.īefore SQL Server 2012, there was no difference between adding a column with the default and adding a column and updating it. In that scenario, still, the above setting will prevail hence the previously set default value will be kept. Now, the next question is what if the Default constraint is dropped just after it is created. When a row is updated, then the default value will be pushed to the table even if the default value column is not updated. So when the column added with a default value, it will not update the data page instead it will update this system table. sys.system_internals_partition_columns DMV has two additional columns named has_default and default_value as shown below. This is achieved by a somewhat very simple but novel approach. In SQL Server 2012 and onwards, this is no longer the situation, the column is added online to the table and no update occurs and it is only a metadata change. What is the difference in these scenarios? Prior to the SQL Server 2012, when adding a column with default value will cause the same behavior. ![]() During the column update, the transaction log will grow and exclusive locking will be placed on the table prohibiting any reads or writes to the table. These stats show that there is a remarkable difference between adding a column with a default value and adding a column and updating the column with a value later. The following is the database file sizes when the column is added and updated the values. ![]() The following are the details for the query expenses captured from the SQL Profiler.Įvidently, when updating a column for a large table, resource consumption is high. The table has grown by some value as shown in the above figure. The above table shows that the table is exclusively locked which means that the table is not accessible during the update. Let us update the same column with a different value and let us get the same stats. All of these results indicate that adding a column with a default constraint will result in only a metadata change. Nothing much has changed to the data file as well as for the log file. Let us see the file sizes of the database. Also, adding a column with default value has not taken even a one minute though it has 500,000 records. This shows that Table (Object) has intended Exclusive lock which means that the table is not exclusively locked during the addition of the column.
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