UPDATE¶
Function¶
Updates data in a table. UPDATE changes the values of the specified columns in all rows that satisfy the condition. The WHERE clause clarifies conditions. The columns to be modified need be mentioned in the SET clause; columns not explicitly modified retain their previous values.
Precautions¶
You must have the UPDATE permission on a table to be updated.
You must have the SELECT permission on all tables involved in the expressions or conditions.
The distribution column of a table cannot be modified.
For column-store tables, the RETURNING clause is currently not supported.
Column-store tables do not support non-deterministic updates. If you update data in one row with multiple rows of data in a column-store table, an error is reported.
Memory space that records update operations in column-store tables is not reclaimed. You need to clean it by executing VACUUM FULL table_name.
You are not advised to create a table that needs to be frequently updated as a replication table.
Column-store tables support lightweight UPDATE operations. Lightweight UPDATE operations only rewrite the updated columns to reduce space usage. Lightweight UPDATE for column-store tables is controlled by GUC parameter enable_light_colupdate. It is disabled by default.
Column-store lightweight UPDATE is unavailable and automatically changes to the regular UPDATE operation in the following scenarios: updating an index column, updating a primary key column, updating a partition column, updating a PCK column, and online scaling.
There is a low probability that an error is reported when lightweight UPDATE and backend column-store AUTOVACUUM coexist. You can run ALTER TABLE to set the table-level parameter enable_column_autovacuum_garbage to off to avoid this issue. If the table-level parameter enable_column_autovacuum_garbage is set to off, the backend column-store AUTOVACUUM of the table is disabled.
Syntax¶
UPDATE [/*+ plan_hint */] [ ONLY ] table_name [ * ] [ [ AS ] alias ]
SET {column_name = { expression | DEFAULT }
|( column_name [, ...] ) = {( { expression | DEFAULT } [, ...] ) |sub_query }}[, ...]
[ FROM from_list] [ WHERE condition ]
[ RETURNING {*
| {output_expression [ [ AS ] output_name ]} [, ...] }];
where sub_query can be:
SELECT [ ALL | DISTINCT [ ON ( expression [, ...] ) ] ]
{ * | {expression [ [ AS ] output_name ]} [, ...] }
[ FROM from_item [, ...] ]
[ WHERE condition ]
[ GROUP BY grouping_element [, ...] ]
[ HAVING condition [, ...] ]
Parameter Description¶
plan_hint clause
Following the keyword in the
/*+ */format, hints are used to optimize the plan generated by a specified statement block. For details, see "Hint-based Tuning" in the Data Warehouse Service (DWS) Developer Guide.table_name
Name (optionally schema-qualified) of the table to be updated.
Value range: an existing table name
alias
Specifies the alias for the target table.
Value range: a string. It must comply with the naming convention.
column_name
Renames a column.
You can refer to this column by specifying the table name and column name of the target table. Example:
UPDATE foo SET foo.col_name = 'GaussDB';
You can refer to this column by specifying the target table alias and the column name. For example:
UPDATE foo AS f SET f.col_name = 'GaussDB';
Value range: an existing column name
expression
An expression or value to assign to the column.
DEFAULT
Sets the column to its default value.
The value is NULL if no specified default value has been assigned to it.
sub_query
Specifies a subquery.
This command can be executed to update a table with information for other tables in the same database. For details about clauses in the SELECT statement, see SELECT.
from_list
A list of table expressions, allowing columns from other tables to appear in the WHERE condition and the update expressions. This is similar to the list of tables that can be specified in the FROM clause of a SELECT statement.
Important
Note that the target table must not appear in the from_list, unless you intend a self-join (in which case it must appear with an alias in the from_list).
condition
An expression that returns a value of type boolean. Only rows for which this expression returns true are updated.
output_expression
An expression to be computed and returned by the UPDATE command after each row is updated.
Value range: The expression can use any column names of the table named by table_name or table(s) listed in FROM. Write * to return all columns.
output_name
A name to use for a returned column.
Examples¶
Update the values of all records.
UPDATE reason SET r_reason_sk = r_reason_sk * 2;
If the WHERE clause is not included, all r_reason_sk values are updated.
UPDATE reason SET r_reason_sk = r_reason_sk + 100;
Redefine r_reason_sk whose r_reason_desc is reason2 in the reason table.
UPDATE reason SET r_reason_sk = 5 WHERE r_reason_desc = 'reason2';
Redefine r_reason_sk whose value is 2 in the reason table.
UPDATE reason SET r_reason_sk = r_reason_sk + 100 WHERE r_reason_sk = 2;
Redefine the course IDs whose r_reason_sk is greater than 2 in the reason table.
UPDATE reason SET r_reason_sk = 201 WHERE r_reason_sk > 2;
You can run an UPDATE statement to update multiple columns by specifying multiple values in the SET clause. For example:
UPDATE reason SET r_reason_sk = 5, r_reason_desc = 'reason5' WHERE r_reason_id = 'fourth';