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what table alias to use on update with self join - Microsoft SQL / MS SQL Server

> I am stcuk between two 'rules' of SQL that prevent me from  What about this: update my_table set flag_col = 'y from my_table, my_table PRIOR where my_table.key = PRIOR.key and my_table.date_col <PRIOR.date_col / Erik...

  1. #1

    Default Re: what table alias to use on update with self join

    > I am stcuk between two 'rules' of SQL that prevent me from 

    What about this:
    update my_table
    set flag_col = 'y
    from my_table, my_table PRIOR
    where my_table.key = PRIOR.key
    and my_table.date_col <PRIOR.date_col

    / Erik


    Erik Guest

  2. #2

    Default Re: what table alias to use on update with self join

    Try using the alias in your UPDATE statemment like...

    update NEWER
    set flag_col = 'y
    from my_table NEWER , my_table PRIOR
    where NEWER.key = PRIOR.key
    and NEWER.date_col <PRIORE.date_col

    HTH,

    Robert




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    Robert Guest

  3. #3

    Default Re: what table alias to use on update with self join


    "larry" <com> wrote in message
    news:024d01c36b40$c2cf2460$gbl... 

    Maybe a stupid question, but:
    When joining a table to itself, how could a field value ever be less than
    itself?

    Perhaps you are trying to join to an older version of the table, in which
    case the second table name would be different.


    Russ Guest

  4. #4

    Default Re: what table alias to use on update with self join

    Your query makes no sense even in T-SQL dialect. Assuming the key_col
    is the key, then for the same row (Newer.key_col = Prior.key_col), you
    will never have (Newer.date_col < Prior.date_col).

    UPDATE MyTable
    SET flag_col = 'Y
    FROM MyTable AS Newer, MyTable AS Prior
    WHERE Newer.key_col = Prior.key_col
    AND Newer.date_col < Prior.date_col;

    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 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
    cardinatliy violation is raised.



    --CELKO--
    ===========================
    Please post DDL, so that people do not have to guess what the keys,
    constraints, Declarative Referential Integrity, datatypes, etc. in your
    schema are.

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