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building a SET clause in shell script
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UPDATE(7) SQL Commands UPDATE(7) NAME
UPDATE - update rows of a table SYNOPSIS
UPDATE [ ONLY ] table [ [ AS ] alias ] SET { column = { expression | DEFAULT } | ( column [, ...] ) = ( { expression | DEFAULT } [, ...] ) } [, ...] [ FROM fromlist ] [ WHERE condition | WHERE CURRENT OF cursor_name ] [ 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 men- tioned 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 addi- tional 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
table The name (optionally schema-qualified) of the table to update. 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 The name of a column in table. 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). 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 [select(7)] 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. cursor_name 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 cur- sor. 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 [declare(7)] for more information about using cursors with WHERE CURRENT OF. 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 or table(s) listed in FROM. 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. If count is 0, no rows matched the condition (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 val- ues defined in the RETURNING list, computed over the row(s) updated by the command. 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 ta- ble(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 pre- dictable. 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'; 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; Change the kind column of the table films in the row on which the cursor c_films is currently positioned: UPDATE films SET kind = 'Dramatic' WHERE CURRENT OF c_films; COMPATIBILITY
This command conforms to the SQL standard, except that the FROM and RETURNING clauses are PostgreSQL extensions. 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. 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. SQL - Language Statements 2010-05-14 UPDATE(7)
