![]() ![]() The basic syntax of an ALTER TABLE command to DROP COLUMN in an existing table is as follows.ĪLTER TABLE table_name DROP COLUMN column_name The basic syntax of an ALTER TABLE command to add a New Column in an existing table is as follows.ĪLTER TABLE table_name ADD column_name datatype You should also use the ALTER TABLE command to add and drop various constraints on an existing table. For example, when running ALTER for a Distributed table, you will also need to run ALTER for the tables on all remote servers.The SQL ALTER TABLE command is used to add, delete or modify columns in an existing table. At the same time, all new queries to the same table will wait while this ALTER is running.įor tables that do not store data themselves (such as Merge and Distributed), ALTER just changes the table structure, and does not change the structure of subordinate tables. In other words, if a long SELECT is running at the time of the ALTER query, the ALTER query will wait for it to complete. The ALTER query blocks all reads and writes for the table. You can use the clickhouse-copier as an alternative to the INSERT SELECT query. If the ALTER query is not sufficient to make the table changes you need, you can create a new table, copy the data to it using the INSERT SELECT query, then switch the tables using the RENAME query and delete the old table. Changing the type for columns that are included in the primary key is only possible if this change does not cause the data to be modified (for example, you are allowed to add values to an Enum or to change a type from DateTime to UInt32). There is no support for deleting columns in the primary key or the sampling key (columns that are used in the ENGINE expression). A nested data structure is equivalent to multiple array columns with a name that has the same prefix before the dot. To add a nested data structure, you can add columns with a name like name.nested_name and the type Array(T). The ALTER query lets you create and delete separate elements (columns) in nested data structures, but not whole nested data structures. SELECT groupArray ( x ), groupArray ( s ) FROM tmp SELECT groupArray ( x ), groupArray ( s ) FROM ( select x, s from tmp order by x ) numbers LIMIT 5 ĪLTER TABLE tmp ADD COLUMN s String MATERIALIZED toString ( x ) This approach allows us to complete the ALTER query instantly, without increasing the volume of old data.ĬREATE TABLE tmp ( x Int64 ) ENGINE = MergeTree ( ) ORDER BY tuple ( ) PARTITION BY tuple ( ) The column appears on the disk after merging data parts (see MergeTree). ![]() If the data is missing for a column when reading from the table, it is filled in with default values (by performing the default expression if there is one, or using zeros or empty strings). The data does not appear on the disk after ALTER. For a chain of actions, name_after can be the name of a column that is added in one of the previous actions.Īdding a column just changes the table structure, without performing any actions with data. Otherwise, the column is added to the end of the table. If you want to add a column to the beginning of the table use the FIRST clause. ![]() If you specify AFTER name_after (the name of another column), the column is added after the specified one in the list of table columns. If the IF NOT EXISTS clause is included, the query won’t return an error if the column already exists. Adds a new column to the table with the specified name, type, codec and default_expr (see the section Default expressions). ![]()
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