Is this DELETE possible?

[php newbie]

I am getting error messages when I try to delete from a table using
the values in the table itself. The intent is to delete all rows from
TableA where col_2 matches any of the col_1 values.

DELETE FROM TableA FROM TableA x INNER JOIN TableA y ON (x.col_1 =
y.col_2)

Error msg: The table 'TableA' is ambiguous.

Can this be done with SQL or should I use T-SQL with cursors here?

[Aaron W. West]

Re: Is this DELETE possible?

Try EXISTS or IN

DELETE TableA
WHERE EXISTS (SELECT * FROM TableA y
WHERE TableA.col_2 = y.col_1)

Or did you want:

DELETE TableA
WHERE EXISTS (SELECT * FROM TableA y
WHERE TableA.col_1 = y.col_2)

As always, I recommend you test with SELECT queries first. (No warrantees
implied, etc...)


"php newbie" <newtophp2000@y ahoo.com> wrote in message
news:124f428e.0 407131933.72eea 682@posting.goo gle.com...
I am getting error messages when I try to delete from a table using
the values in the table itself. The intent is to delete all rows from
TableA where col_2 matches any of the col_1 values.

DELETE FROM TableA FROM TableA x INNER JOIN TableA y ON (x.col_1 =
y.col_2)

Error msg: The table 'TableA' is ambiguous.

Can this be done with SQL or should I use T-SQL with cursors here?

[Dan Guzman]

Re: Is this DELETE possible?

Try:

DELETE FROM x
FROM TableA x
INNER JOIN TableA y ON (x.col_1 = y.col_2)

--
Hope this helps.

Dan Guzman
SQL Server MVP

"php newbie" <newtophp2000@y ahoo.com> wrote in message
news:124f428e.0 407131933.72eea 682@posting.goo gle.com...[color=blue]
> I am getting error messages when I try to delete from a table using
> the values in the table itself. The intent is to delete all rows from
> TableA where col_2 matches any of the col_1 values.
>
> DELETE FROM TableA FROM TableA x INNER JOIN TableA y ON (x.col_1 =
> y.col_2)
>
> Error msg: The table 'TableA' is ambiguous.
>
> Can this be done with SQL or should I use T-SQL with cursors here?[/color]

[php newbie]

Re: Is this DELETE possible?

Aaron,

Thanks for the tip. It worked!

On a related note, I am facing the same error when I try to update
TableA with data from the same TableA.

The command I used is this:

UPDATE TableA SET col_2 = y.col_2
FROM TableA x INNER JOIN TableA y
ON (x.col_1 = y.col_1)

Error message is: The table 'TableA' is ambiguous.

Do you have a similar solution?



"Aaron W. West" <tallpeak@hotma il.NO.SPAM> wrote in message news:<P4udnTGzy aWGIWndRVn-hA@speakeasy.ne t>...[color=blue]
> Try EXISTS or IN
>
> DELETE TableA
> WHERE EXISTS (SELECT * FROM TableA y
> WHERE TableA.col_2 = y.col_1)
>
> As always, I recommend you test with SELECT queries first. (No warrantees
> implied, etc...)[/color]

[Ross Presser]

Re: Is this DELETE possible?

On 14 Jul 2004 07:59:43 -0700, php newbie wrote:
[color=blue]
> Aaron,
>
> Thanks for the tip. It worked!
>
> On a related note, I am facing the same error when I try to update
> TableA with data from the same TableA.
>
> The command I used is this:
>
> UPDATE TableA SET col_2 = y.col_2
> FROM TableA x INNER JOIN TableA y
> ON (x.col_1 = y.col_1)
>
> Error message is: The table 'TableA' is ambiguous.
>
> Do you have a similar solution?
>[/color]

If you're using "x" as a label for TableA, you need to use it throughout.

UPDATE x SET x.col_2 = y.col_2
FROM TableA x
INNER JOIN TableA y
ON (x.col_1 = y.col_1)

[Jim Geissman]

Re: Is this DELETE possible?

I also got it to work like this:

CREATE TABLE #T (A int,B int)
INSERT #T SELECT 1,2
INSERT #T SELECT 2,2
INSERT #T SELECT 3,3
INSERT #T SELECT 4,3
INSERT #T SELECT 5,3
INSERT #T SELECT 6,4
INSERT #T SELECT 7,4

SELECT * FROM #T WHERE A IN (SELECT DISTINCT B FROM #T)

DELETE FROM #T WHERE A IN (SELECT DISTINCT B FROM #T)

[--CELKO--]

Re: Is this DELETE possible?

>> On a related note, I am facing the same error when I try to update
TableA with data from the same TableA. <<

Why in the world do you think that SQL has a FROM clause in UPDATE and
DELETE??? You are writing unpredictable, proprietary code that EVEN
IF IT WAS ALLOWED, would not produce results.

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. And when Microsoft decides to change it, you will have to do
a re-write. Remember the deprecated "*=" versus "LEFT OUTER JOIN"
conversions? The last time the UPDATE FROM changed?

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, the result set is cast to a NULL; if it returns multiple rows,
a cardinality violation is raised.

Same logic for the basic delete statement:

DELETE FROM Foobar
WHERE EXISTS
(SELECT *
FROM Foobar AS F1
WHERE Foobar.col_1 = F1.col_2)

[php newbie]

Re: Is this DELETE possible?

Dan,

This works beautifully and also applies directly to the update query. Thanks a lot!

My gratitudes also go to Russ and Jim. I appreciate your help.


"Dan Guzman" <danguzman@nosp am-earthlink.net> wrote in message news:<kZ9Jc.287 9$Qu5.1593@news read2.news.pas. earthlink.net>. ..[color=blue]
> Try:
>
> DELETE FROM x
> FROM TableA x
> INNER JOIN TableA y ON (x.col_1 = y.col_2)
>
> --
> Hope this helps.
>
> Dan Guzman
> SQL Server MVP[/color]

[vic]

Re: Is this DELETE possible?

newtophp2000@ya hoo.com (php newbie) wrote in message news:<124f428e. 0407131933.72ee a682@posting.go ogle.com>...[color=blue]
> I am getting error messages when I try to delete from a table using
> the values in the table itself. The intent is to delete all rows from
> TableA where col_2 matches any of the col_1 values.
>
> DELETE FROM TableA FROM TableA x INNER JOIN TableA y ON (x.col_1 =
> y.col_2)
>
> Error msg: The table 'TableA' is ambiguous.
>
> Can this be done with SQL or should I use T-SQL with cursors here?[/color]

Hi,
try this:
DELETE x FROM TableA AS x INNER JOIN TableA AS y ON
(x.col_1 = y.col_2)

With best regards!

[vic]

Re: Is this DELETE possible?

newtophp2000@ya hoo.com (php newbie) wrote in message news:<124f428e. 0407131933.72ee a682@posting.go ogle.com>...[color=blue]
> I am getting error messages when I try to delete from a table using
> the values in the table itself. The intent is to delete all rows from
> TableA where col_2 matches any of the col_1 values.
>
> DELETE FROM TableA FROM TableA x INNER JOIN TableA y ON (x.col_1 =
> y.col_2)
>
> Error msg: The table 'TableA' is ambiguous.
>
> Can this be done with SQL or should I use T-SQL with cursors here?[/color]

Hi,
try this:
DELETE x FROM TableA AS x INNER JOIN TableA AS y ON
(x.col_1 = y.col_2)

With best regards!

[Dan Guzman]

Re: Is this DELETE possible?

> Remember the deprecated "*=" versus "LEFT OUTER JOIN"[color=blue]
> conversions?[/color]

Joe,

Just out of curiosity, was LEFT OUTER JOIN (et. al.) always part of the ANSI
standard? If so, why do you think major database vendors such as Microsoft,
Sybase and Oracle choose proprietary syntax?

--
Hope this helps.

Dan Guzman
SQL Server MVP

"--CELKO--" <jcelko212@eart hlink.net> wrote in message
news:18c7b3c2.0 407141855.550ab a73@posting.goo gle.com...[color=blue][color=green][color=darkred]
> >> On a related note, I am facing the same error when I try to update[/color][/color]
> TableA with data from the same TableA. <<
>
> Why in the world do you think that SQL has a FROM clause in UPDATE and
> DELETE??? You are writing unpredictable, proprietary code that EVEN
> IF IT WAS ALLOWED, would not produce results.
>
> 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. And when Microsoft decides to change it, you will have to do
> a re-write. Remember the deprecated "*=" versus "LEFT OUTER JOIN"
> conversions? The last time the UPDATE FROM changed?
>
> 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, the result set is cast to a NULL; if it returns multiple rows,
> a cardinality violation is raised.
>
> Same logic for the basic delete statement:
>
> DELETE FROM Foobar
> WHERE EXISTS
> (SELECT *
> FROM Foobar AS F1
> WHERE Foobar.col_1 = F1.col_2)[/color]

[Dan Guzman]

Re: Is this DELETE possible?

Glad it helped.

--
Dan Guzman
SQL Server MVP

"php newbie" <newtophp2000@y ahoo.com> wrote in message
news:124f428e.0 407141856.659b7 0a8@posting.goo gle.com...[color=blue]
> Dan,
>
> This works beautifully and also applies directly to the update query.[/color]
Thanks a lot![color=blue]
>
> My gratitudes also go to Russ and Jim. I appreciate your help.
>
>[/color]

[VC]

Re: Is this DELETE possible?

Dan,

Both implementations pre-dated the '92 standard.

VC

"Dan Guzman" <danguzman@nosp am-earthlink.net> wrote in message
news:%vHJc.4441 $Qu5.433@newsre ad2.news.pas.ea rthlink.net...[color=blue][color=green]
> > Remember the deprecated "*=" versus "LEFT OUTER JOIN"
> > conversions?[/color]
>
> Joe,
>
> Just out of curiosity, was LEFT OUTER JOIN (et. al.) always part of the[/color]
ANSI[color=blue]
> standard? If so, why do you think major database vendors such as[/color]
Microsoft,[color=blue]
> Sybase and Oracle choose proprietary syntax?
>
> --
> Hope this helps.
>
> Dan Guzman
> SQL Server MVP
>
> "--CELKO--" <jcelko212@eart hlink.net> wrote in message
> news:18c7b3c2.0 407141855.550ab a73@posting.goo gle.com...[color=green][color=darkred]
> > >> On a related note, I am facing the same error when I try to update[/color]
> > TableA with data from the same TableA. <<
> >
> > Why in the world do you think that SQL has a FROM clause in UPDATE and
> > DELETE??? You are writing unpredictable, proprietary code that EVEN
> > IF IT WAS ALLOWED, would not produce results.
> >
> > 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. And when Microsoft decides to change it, you will have to do
> > a re-write. Remember the deprecated "*=" versus "LEFT OUTER JOIN"
> > conversions? The last time the UPDATE FROM changed?
> >
> > 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, the result set is cast to a NULL; if it returns multiple rows,
> > a cardinality violation is raised.
> >
> > Same logic for the basic delete statement:
> >
> > DELETE FROM Foobar
> > WHERE EXISTS
> > (SELECT *
> > FROM Foobar AS F1
> > WHERE Foobar.col_1 = F1.col_2)[/color]
>
>[/color]

[Dan Guzman]

Re: Is this DELETE possible?

> Both implementations pre-dated the '92 standard.

So it appears many vendors implemented proprietary SQL extensions to address
deficiencies in the SQL-89 standard. Once the standard was enhanced to
address the need, many vendors added support for ANSI-style joins as well.

Portability is a consideration but, IMHO, is less important than
functionality in most environments.

--
Hope this helps.

Dan Guzman
SQL Server MVP

"VC" <boston103@hotm ail.com> wrote in message
news:LuPJc.8724 0$JR4.26140@att bi_s54...[color=blue]
> Dan,
>
> Both implementations pre-dated the '92 standard.
>
> VC
>
> "Dan Guzman" <danguzman@nosp am-earthlink.net> wrote in message
> news:%vHJc.4441 $Qu5.433@newsre ad2.news.pas.ea rthlink.net...[color=green][color=darkred]
> > > Remember the deprecated "*=" versus "LEFT OUTER JOIN"
> > > conversions?[/color]
> >
> > Joe,
> >
> > Just out of curiosity, was LEFT OUTER JOIN (et. al.) always part of the[/color]
> ANSI[color=green]
> > standard? If so, why do you think major database vendors such as[/color]
> Microsoft,[color=green]
> > Sybase and Oracle choose proprietary syntax?
> >
> > --
> > Hope this helps.
> >
> > Dan Guzman
> > SQL Server MVP
> >
> > "--CELKO--" <jcelko212@eart hlink.net> wrote in message
> > news:18c7b3c2.0 407141855.550ab a73@posting.goo gle.com...[color=darkred]
> > > >> On a related note, I am facing the same error when I try to update
> > > TableA with data from the same TableA. <<
> > >
> > > Why in the world do you think that SQL has a FROM clause in UPDATE and
> > > DELETE??? You are writing unpredictable, proprietary code that EVEN
> > > IF IT WAS ALLOWED, would not produce results.
> > >
> > > 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. And when Microsoft decides to change it, you will have to do
> > > a re-write. Remember the deprecated "*=" versus "LEFT OUTER JOIN"
> > > conversions? The last time the UPDATE FROM changed?
> > >
> > > 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, the result set is cast to a NULL; if it returns multiple rows,
> > > a cardinality violation is raised.
> > >
> > > Same logic for the basic delete statement:
> > >
> > > DELETE FROM Foobar
> > > WHERE EXISTS
> > > (SELECT *
> > > FROM Foobar AS F1
> > > WHERE Foobar.col_1 = F1.col_2)[/color]
> >
> >[/color]
>
>[/color]

[Erland Sommarskog]

Re: Is this DELETE possible?

--CELKO-- (jcelko212@eart hlink.net) writes:[color=blue]
> There is no FROM clause in a Standard SQL UPDATE statement; it would
> make no sense.[/color]

Of course it would.
[color=blue]
> Other programmers cannot read it and maintaining it is harder.[/color]

Au contraire, I find nested subselects more difficult to understand
and maintain.

So you have this UPDATE statement:

UPDATE tblA
SET col1 = z.col1
col2 = y.col2
FROM tblA a
JOIN ...
WHERE ...

And we are interested in which rows this statement actually hits. A little
cut and paste, select "UPDATE tblA SET", type SELECT, press Execute et
voilà!


--
Erland Sommarskog, SQL Server MVP, esquel@sommarsk og.se

Books Online for SQL Server SP3 at

http://www.microsoft.com/sql/techinfo/productdoc/2000/books.asp