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.alter table command

  • Article

Applies to: ✅ Microsoft Fabric ✅ Azure Data Explorer

The .alter table command:

  • Secures data in "preserved" columns
  • Reorders table columns
  • Sets a new column schema, docstring, and folder to an existing table, overwriting the existing column schema, docstring, and folder
  • Must run in the context of a specific database that scopes the table name

Warning

Using the .alter command incorrectly may lead to data loss.

Permissions

You must have at least Table Admin permissions to run this command.

Syntax

.alter table tableName (columnName:columnType [, ...]) [with (propertyName = propertyValue [, ...])]

Learn more about syntax conventions.

Parameters

Name Type Required Description
tableName string ✔️ The name of the table to alter.
columnName, columnType string ✔️ The name of an existing or new column mapped to the type of data in that column. The list of these mappings defines the output column schema.
propertyName, propertyValue string A comma-separated list of key-value property pairs. See supported properties.

Warning

Existing columns that aren't specified in the command will be dropped. This could lead to unexpected data loss.

Tip

Use .show table [tableName] cslschema to get the existing table schema before you alter it.

Supported properties

Name Type Description
docstring string Free text describing the entity to be added. This string is presented in various UX settings next to the entity names.
folder string The name of the folder to add to the table.

How the command affects the data

  • Existing data in columns listed in the command won't be modified
  • Existing data in columns not listed in the command will be deleted
  • New columns will be added to the end of the schema
  • Data in new columns is assumed to be null
  • The table will have the same columns, in the same order, as specified

Note

If you try to alter a column type, the command will fail. Use .alter column instead.

Warning

  • Data ingestion that disregards the order of columns and occurs in parallel with .alter table risks ingesting data into the wrong columns. To prevent this, make sure that ingestion uses a mapping object or stop ingestion while running the .alter table command.
  • Data ingestion may modify a table's column schema. Be careful not to accidentally remove desired columns that were added during ingestion.

Examples

.alter table MyTable (ColumnX:string, ColumnY:int) 
.alter table MyTable (ColumnX:string, ColumnY:int) with (docstring = "Some documentation", folder = "Folder1")

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