update.7

PostgreSQL 8.2中增加了很多企业用户所需要的功能和性能上的提高,其开发团队说,该版本将加速更多企业向该数据库移植.核心开发成员之一Bruce Momjian表示,在新版PostgreSQL

.\\" auto-generated by docbook2man-spec $Revision: 1.1.1.1 $
.TH "UPDATE" "" "2008-01-03" "SQL - Language Statements" "SQL Commands"
.SH NAME
UPDATE \- update rows of a table

.SH SYNOPSIS
.sp
.nf
UPDATE [ ONLY ] \fItable\fR [ [ AS ] \fIalias\fR ]
    SET { \fIcolumn\fR = { \fIexpression\fR | DEFAULT } |
          ( \fIcolumn\fR [, ...] ) = ( { \fIexpression\fR | DEFAULT } [, ...] ) } [, ...]
    [ FROM \fIfromlist\fR ]
    [ WHERE \fIcondition\fR ]
    [ RETURNING * | \fIoutput_expression\fR [ AS \fIoutput_name\fR ] [, ...] ]
.sp
.fi
.SH "DESCRIPTION"
.PP
\fBUPDATE\fR 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.
.PP
By default, \fBUPDATE\fR 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.
.PP
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.
.PP
The optional RETURNING clause causes \fBUPDATE\fR
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 \fBSELECT\fR.
.PP
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
\fIexpressions\fR or
\fIcondition\fR.
.SH "PARAMETERS"
.TP
\fB\fItable\fB\fR
The name (optionally schema-qualified) of the table to update.
.TP
\fB\fIalias\fB\fR
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
\fBUPDATE\fR statement must refer to this table as
f not foo.
.TP
\fB\fIcolumn\fB\fR
The name of a column in \fItable\fR.
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 \(em for example,
UPDATE tab SET tab.col = 1 is invalid.
.TP
\fB\fIexpression\fB\fR
An expression to assign to the column. The expression may use the
old values of this and other columns in the table.
.TP
\fBDEFAULT\fR
Set the column to its default value (which will be NULL if no
specific default expression has been assigned to it).
.TP
\fB\fIfromlist\fB\fR
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 [\fBselect\fR(7)] of a \fBSELECT\fR
statement. Note that the target table must not appear in the
\fIfromlist\fR, unless you intend a self-join (in which
case it must appear with an alias in the \fIfromlist\fR).
.TP
\fB\fIcondition\fB\fR
An expression that returns a value of type \fBboolean\fR.
Only rows for which this expression returns true
will be updated.
.TP
\fB\fIoutput_expression\fB\fR
An expression to be computed and returned by the \fBUPDATE\fR
command after each row is updated. The expression may use any
column names of the \fItable\fR
or table(s) listed in FROM.
Write * to return all columns.
.TP
\fB\fIoutput_name\fB\fR
A name to use for a returned column.
.SH "OUTPUTS"
.PP
On successful completion, an \fBUPDATE\fR command returns a command
tag of the form
.sp
.nf
UPDATE \fIcount\fR
.sp
.fi
The \fIcount\fR is the number
of rows updated. If \fIcount\fR is
0, no rows matched the \fIcondition\fR (this is not considered
an error).
.PP
If the \fBUPDATE\fR command contains a RETURNING
clause, the result will be similar to that of a \fBSELECT\fR
statement containing the columns and values defined in the
RETURNING list, computed over the row(s) updated by the
command.
.SH "NOTES"
.PP
When a FROM clause is present, what essentially happens
is that the target table is joined to the tables mentioned in the
\fIfromlist\fR, 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.
.PP
Because of this indeterminacy, referencing other tables only within
sub-selects is safer, though often harder to read and slower than
using a join.
.SH "EXAMPLES"
.PP
Change the word Drama to Dramatic in the
column \fBkind\fR of the table \fBfilms\fR:
.sp
.nf
UPDATE films SET kind = 'Dramatic' WHERE kind = 'Drama';
.sp
.fi
.PP
Adjust temperature entries and reset precipitation to its default
value in one row of the table \fBweather\fR:
.sp
.nf
UPDATE weather SET temp_lo = temp_lo+1, temp_hi = temp_lo+15, prcp = DEFAULT
  WHERE city = 'San Francisco' AND date = '2003-07-03';
.sp
.fi
.PP
Perform the same operation and return the updated entries:
.sp
.nf
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;
.sp
.fi
.PP
Use the alternative column-list syntax to do the same update:
.sp
.nf
UPDATE weather SET (temp_lo, temp_hi, prcp) = (temp_lo+1, temp_lo+15, DEFAULT)
  WHERE city = 'San Francisco' AND date = '2003-07-03';
.sp
.fi
.PP
Increment the sales count of the salesperson who manages the
account for Acme Corporation, using the FROM
clause syntax:
.sp
.nf
UPDATE employees SET sales_count = sales_count + 1 FROM accounts
  WHERE accounts.name = 'Acme Corporation'
  AND employees.id = accounts.sales_person;
.sp
.fi
.PP
Perform the same operation, using a sub-select in the
WHERE clause:
.sp
.nf
UPDATE employees SET sales_count = sales_count + 1 WHERE id =
  (SELECT sales_person FROM accounts WHERE name = 'Acme Corporation');
.sp
.fi
.PP
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.
.sp
.nf
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;
.sp
.fi
.SH "COMPATIBILITY"
.PP
This command conforms to the SQL standard, except
that the FROM and RETURNING clauses
are PostgreSQL extensions.
.PP
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:
.sp
.nf
UPDATE accounts SET (contact_last_name, contact_first_name) =
    (SELECT last_name, first_name FROM salesmen
     WHERE salesmen.id = accounts.sales_id);
.sp
.fi
This is not currently implemented \(em the source must be a list
of independent expressions.
.PP
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.