PostgreSQL 8.1.8 Documentation
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# UPDATE

## Name

UPDATE -- update rows of a table

## Synopsis

    UPDATE [ ONLY ] table SET column = { expression | DEFAULT } [, ...]
        [ FROM fromlist ]
        [ WHERE condition ]

## 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.

By default, `UPDATE` will update rows in the specified table and all its subtables. If you wish to only update the specific table mentioned, you must use the `ONLY` clause.

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.

You must have the `UPDATE` privilege on the table to update it, as well as the `SELECT` privilege to any table whose values are read in the `expression`s or `condition`.

## Parameters

`table`

  The name (optionally schema-qualified) of the table to update.

`column`

  The name of a column in `table`. The column name can be qualified with a subfield name or array subscript, if needed.

`expression`

  An expression to assign to the column. The expression may 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).

`fromlist`

  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 `fromlist`, unless you intend a self-join (in which case it must appear with an alias in the `fromlist`).

`condition`

  An expression that returns a value of type `boolean`. Only rows for which this expression returns `true` will be updated.

## Outputs

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

    UPDATE count

The `count` is the number of rows updated. If `count` is 0, no rows matched the `condition` (this is not considered an error).

## Notes

When a `FROM` clause is present, what essentially happens is that the target table is joined to the tables mentioned in the `fromlist`, and each output row of the join represents an update operation for the target table. When using `FROM` you should ensure that the join produces at most one output row for each row to be modified. In other words, a target row shouldn't join to more than one row from the other table(s). If it does, then only one of the join rows will be used to update the target row, but which one will be used is not readily predictable.

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';

Increment the sales count of the salesperson who manages the account for Acme Corporation, using the `FROM` clause syntax:

    UPDATE employees SET sales_count = sales_count + 1 FROM accounts
      WHERE accounts.name = 'Acme Corporation'
      AND employees.id = accounts.sales_person;

Perform the same operation, using a sub-select in the `WHERE` clause:

    UPDATE employees SET sales_count = sales_count + 1 WHERE id =
      (SELECT sales_person FROM accounts WHERE name = 'Acme Corporation');

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` clause is a PostgreSQL extension.

Some other database systems offer a `FROM` option in which the target table is supposed to be listed again within `FROM`. That is not how PostgreSQL interprets `FROM`. Be careful when porting applications that use this extension.

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