>> [UPDATE with FROM clause] it's very standard SQL (not that that
means anything to Oracle). <<
Nope! There is no FROM clause in a Standard SQL UPDATE statement; it
would make no sense. Other products (SQL Server, Sybase and Ingres)
also use the UPDATE .. FROM syntax, but with **different** semantics.
So it does not port, or even worse, when you do move it, it trashes
your database. Other programmers cannot read it and maintaining it is
harder.
The correct syntax for a searched update statement is
<update statement> ::=
UPDATE <table name>
SET <set clause list>
[WHERE <search condition>]
<set clause list> ::=
<set clause> [{ , <set clause> }...]
<set clause> ::= <object column> = <update source>
<update source> ::= <value expression> | NULL | DEFAULT
<object column> ::= <column name>
The UPDATE clause simply gives the name of the base table or updatable
view to be changed.
Notice that no correlation name is allowed in the UPDATE clause; this
is to avoid some self-referencing problems that could occur. But it
also follows the data model in Standard SQL. When you give a table
expression a correlation name, it is to act as if a materialized table
with that correlation name has been created in the database. That
table then is dropped at the end of the statement. If you allowed
correlation names in the UPDATE clause, you would be updating the
materialized table, which would then disappear and leave the base
table untouched.
The SET clause is a list of columns to be changed or made; the WHERE
clause tells the statement which rows to use. For this discussion, we
will assume the user doing the update has applicable UPDATE privileges
for each <object column>.
* The WHERE Clause
As mentioned, the most important thing to remember about the WHERE
clause is that it is optional. If there is no WHERE clause, all rows
in the table are changed. This is a common error; if you make it,
immediately execute a ROLLBACK statement.
All rows that test TRUE for the <search condition> are marked as a
subset and not as individual rows. It is also possible that this
subset will be empty. This subset is used to construct a new set of
rows that will be inserted into the table when the subset is deleted
from the table. Note that the empty subset is a valid update that
will fire declarative referential actions and triggers.
* The SET Clause
Each assignment in the <set clause list> is executed in parallel and
each SET clause changes all the qualified rows at once. Or at least
that is the theoretical model. In practice, implementations will
first mark all of the qualified rows in the table in one pass, using
the WHERE clause. If there were no problems, then the SQL engine
makes a copy of each marked row in working storage. Each SET clause
is executed based on the old row image and the results are put in the
new row image. Finally, the old rows are deleted and the new rows are
inserted. If an error occurs during all of this, then system does a
ROLLBACK, the table is left unchanged and the errors are reported.
This parallelism is not like what you find in a traditional
third-generation programming language, so it may be hard to learn.
This feature lets you write a statement that will swap the values in
two columns, thus:
UPDATE MyTable
SET a = b, b = a;
This is not the same thing as
BEGIN ATOMIC
UPDATE MyTable
SET a = b;
UPDATE MyTable
SET b = a;
END;
In the first UPDATE, columns a and b will swap values in each row. In
the second pair of UPDATEs, column a will get all of the values of
column b in each row. In the second UPDATE of the pair, a, which now
has the same value as the original value of b, will be written back
into column b -- no change at all. There are some limits as to what
the value expression can be. The same column cannot appear more than
once in a <set clause list> -- which makes sense, given the parallel
nature of the statement. Since both go into effect at the same time,
you would not know which SET clause to use.
If a subquery expression is used in a <set clause>, and it returns a
single value, the result set is cast to a scalar; if it returns an
empty set, the result set is cast to a NULL; if it returns multiple
rows, a cardinality violation is raised.
JOINS should never be done in the WHERE clause. That's an archaic
construction. They should be done using the appropriate JOIN syntax;
this keeps the distinction between joins and restricts clear. <<
A lot of newbies (read: SQL Server by way of ACCESS) think that the ON
clause must hold the join condition and the WHERE clause must hold the
restrictions. This is not true. It can make for some nice code, and
might help a smart optimizer materialize common subexpressions across
mulitple queries, show you what should be in a VIEW, etc. But it is
not required.
.. but it isn't logical to write Value = COLUMN. <<
My Arab and Chinese students used to do that, too!
