Altering Metadata

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Most changes to data definitions are made at the table level, and involve adding new columns to a table, or dropping obsolete columns from it. SQL provides ALTER TABLE to add new columns to a table and to drop existing columns. A single ALTER TABLE can carry out a single operation, or both operations. Direct metadata operations such as ALTER TABLE increment the metadata version. Any table (and its triggers) can be modified up to a maximum of 255 times before you must back up and restore the database.

Making changes to views and indexes always requires two separate statements:

  1. Drop the existing definition.
  2. Create a new definition.

If current metadata cannot be dropped, replacement definitions cannot be added. Dropping metadata can fail for the following reasons:

  • The person attempting to drop metadata is not the metadata creator.
  • SQL integrity constraints are defined for the metadata and referenced in other metadata.
  • The metadata is used in another view, trigger, or computed column.

For more information about dropping metadata, see Dropping Metadata.

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