Postgres-XC 1.1 Documentation
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# UPDATE

## Name

UPDATE -- update rows of a table

## Synopsis

    UPDATE [ ONLY ] table_name [ * ] [ [ AS ] alias ]
        SET { column_name = { expression | DEFAULT } |
              ( column_name [, ...] ) = ( { expression | DEFAULT } [, ...] ) } [, ...]
        [ WHERE condition ]
        [ RETURNING * | output_expression [ [ AS ] output_name ] [, ...] ]

## Description

`UPDATE` changes the values of the specified columns in all rows that satisfy the condition. Only the columns to be modified need be mentioned in the `SET` clause; columns not explicitly modified retain their previous values.

There are two ways to modify a table using information contained in other tables in the database: using sub-selects, or specifying additional tables in the `FROM` clause. Which technique is more appropriate depends on the specific circumstances.

The optional `RETURNING` clause causes `UPDATE` to compute and return value(s) based on each row actually updated. Any expression using the table's columns, and/or columns of other tables mentioned in `FROM`, can be computed. The new (post-update) values of the table's columns are used. The syntax of the `RETURNING` list is identical to that of the output list of `SELECT`.

You must have the `UPDATE` privilege on the table, or at least on the column(s) that are listed to be updated. You must also have the `SELECT` privilege on any column whose values are read in the `expressions` or `condition`.

## Parameters

`with_query`

  The `WITH` clause allows you to specify one or more subqueries that can be referenced by name in the `UPDATE` query. See Section 7.8 and SELECT for details.

`table_name`

  The name (optionally schema-qualified) of the table to update. If `ONLY` is specified before the table name, matching rows are updated in the named table only. If `ONLY` is not specified, matching rows are also updated in any tables inheriting from the named table. Optionally, `*` can be specified after the table name to explicitly indicate that descendant tables are included.

`alias`

  A substitute name for the target table. When an alias is provided, it completely hides the actual name of the table. For example, given `UPDATE foo AS f`, the remainder of the `UPDATE` statement must refer to this table as `f` not `foo`.

`column_name`

  The name of a column in the table named by `table_name`. The column name can be qualified with a subfield name or array subscript, if needed. Do not include the table's name in the specification of a target column — for example, `UPDATE tab SET tab.col = 1` is invalid.

`expression`

  An expression to assign to the column. The expression can use the old values of this and other columns in the table.

`DEFAULT`

  Set the column to its default value (which will be NULL if no specific default expression has been assigned to it).

`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. 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` will be updated.

`cursor_name`

  > Note: The following description applies only to PostgreSQL

  The name of the cursor to use in a `WHERE CURRENT OF` condition. The row to be updated is the one most recently fetched from this cursor. The cursor must be a non-grouping query on the `UPDATE`'s target table. Note that `WHERE CURRENT OF` cannot be specified together with a Boolean condition. See DECLARE for more information about using cursors with `WHERE CURRENT OF`. [1]

`output_expression`

  An expression to be computed and returned by the `UPDATE` command after each row is updated. The expression can use any column names of the table named by `table_name`. Write `*` to return all columns.

`output_name`

  A name to use for a returned column.

## Outputs

On successful completion, an `UPDATE` command returns a command tag of the form

    UPDATE count

The `count` is the number of rows updated, including matched rows whose values did not change. Note that the number may be less than the number of rows that matched the `condition` when updates were suppressed by a `BEFORE UPDATE` trigger. If `count` is 0, no rows were updated by the query (this is not considered an error).

If the `UPDATE` command contains a `RETURNING` clause, the result will be similar to that of a `SELECT` statement containing the columns and values defined in the `RETURNING` list, computed over the row(s) updated by the command.

## Notes

Because of this indeterminacy, referencing other tables only within sub-selects is safer, though often harder to read and slower than using a join.

## Examples

Change the word `Drama` to `Dramatic` in the column `kind` of the table `films`:

    UPDATE films SET kind = 'Dramatic' WHERE kind = 'Drama';

Adjust temperature entries and reset precipitation to its default value in one row of the table `weather`:

    UPDATE weather SET temp_lo = temp_lo+1, temp_hi = temp_lo+15, prcp = DEFAULT
      WHERE city = 'San Francisco' AND date = '2003-07-03';

Perform the same operation and return the updated entries:

    UPDATE weather SET temp_lo = temp_lo+1, temp_hi = temp_lo+15, prcp = DEFAULT
      WHERE city = 'San Francisco' AND date = '2003-07-03'
      RETURNING temp_lo, temp_hi, prcp;

Use the alternative column-list syntax to do the same update:

    UPDATE weather SET (temp_lo, temp_hi, prcp) = (temp_lo+1, temp_lo+15, DEFAULT)
      WHERE city = 'San Francisco' AND date = '2003-07-03';

Attempt to insert a new stock item along with the quantity of stock. If the item already exists, instead update the stock count of the existing item. To do this without failing the entire transaction, use savepoints:

    BEGIN;
    -- other operations
    SAVEPOINT sp1;
    INSERT INTO wines VALUES('Chateau Lafite 2003', '24');
    -- Assume the above fails because of a unique key violation,
    -- so now we issue these commands:
    ROLLBACK TO sp1;
    UPDATE wines SET stock = stock + 24 WHERE winename = 'Chateau Lafite 2003';
    -- continue with other operations, and eventually
    COMMIT;

## Compatibility

This command conforms to the SQL standard, except that the `FROM` and `RETURNING` clauses are PostgreSQL extensions, as is the ability to use `WITH` with `UPDATE`.

According to the standard, the column-list syntax should allow a list of columns to be assigned from a single row-valued expression, such as a sub-select:

    UPDATE accounts SET (contact_last_name, contact_first_name) =
        (SELECT last_name, first_name FROM salesmen
         WHERE salesmen.id = accounts.sales_id);

This is not currently implemented — the source must be a list of independent expressions.

## Notes

[1] | `WHERE CURRENT OF` clause is not supported by the current release of Postgres-XC.

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