Modifying existing SQL tables is a common use case as the data models for your application change over time. The most basic modification you are likely to do is adding a new column to a SQL table to capture more data.
Simple changes can be done using SQL Server management software that provides a convenient user interface to interact with your database and tables. For more complex operations, using scripted commands in the Transact-SQL (T-SQL) language is usually preferable. Using T-SQL provides the greatest flexibility to fine-tune the table adjustments, and it makes it very easy to keep a record of the changes made to the table.
To add a column to an existing SQL Server table we are going to use the ALTER keyword. ALTER does more than just allowing us to add columns to a table, it is also used to delete or modify existing columns. It can further be used to add, modify or drop foreign keys on the table columns, as well as modifying constraints like UNIQUE or NOT NULL that can be placed on columns to protect data integrity.
The quickest way to add a new column is without any additional configuration:
Where the datatype is replaced with any valid data type as listed here by Microsoft.
We can append additional configuration to this basic command to define specific requirements for the new column.
Here we are placing a UNIQUE constraint to enforce unique values in the new column:
Adding a column that may not contain NULL values is a bit more complicated. We have to satisfy the NOT NULL constraint right away, meaning we can't create a column that can't be null without providing values for it. The solution is to provide a DEFAULT value:
The chosen default_value depends on the datatype of the column being added and the requirements of your application, e.g. '0' for a column of strings.
Finally, let's add a column of auto-incrementing integers. For this example we use the IDENTITY keyword, which requires a seed value for the first row and an an increment to number the following rows. Both the seed and the increment are set to 1 here, the result is a new column containing values starting 1 to the number of rows in the table.
The IDENTITY keyword does the magic to make the column auto-increment, but be aware that there can only be one identity column per table. Due to its convenient set up and inherent uniqueness, the identity column is commonly used as the PRIMARY KEY column of the table, although this is not mandatory.
Two more constraints you may want to apply while adding a new SQL Server column are FOREIGN KEY and CHECK. FOREIGN KEY is used when a column needs to connect a relationship to another column in a different table, for example to only allow orders against a known product id for an online shop. The CHECK constraint is used to evaluate column entries against a predefined rule - e.g. value must be larger than 0 for a quantity or price column.
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