Understanding SQL

Below is the SQL Statement I am trying to alter.  Can someone help me under stand what the
FROM portion is stating.

SELECT
      B.ORDER.DATE,
      B.ITEM.DESCRIPTION,
      B.QUANTITY,
      B.AMOUNT.BILLED,
      B.CANCEL.DATE,
      B.TYPE.OF.CANCEL,
      B.DAL.CANCELLED.BY,
      B.DAL.REASON.NOTES,
      B.DAL.REASON.TEXT,
      B.BILL.DATE,
FROM
      BILLING.TRANSACTIONS AS B LEFT OUTER JOIN DAILY.ACTIVITY.LOG AS D ON B.@ID = CAST
      (D.BILLING.TRANSACTION.NUM AS VARCHAR)
WHERE
      B.CANCEL.DATE = CURRENT_DATE
      AND B.CANCEL.DATE > CURRENT_DATE - 1
garyrobbinsAsked:
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Scott PletcherSenior DBACommented:
The SQL's not valid.  The @ in that location is not valid, and the extra '.'s in the column names are not valid.

Other than the syntax errors, though, it's a standard LEFT JOIN: from table1 left join to table2.
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Dave FordSoftware Developer / Database AdministratorCommented:
Scott's right. The syntax is slightly missed up, but what exactly are you asking?

Do you understand LEFT OUTER JOIN?
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PortletPaulfreelancerCommented:
There is also an "incomplete query" issue because your select clause ends in a comma

If I assumed all of the illegal . are underscores _
e.g.
B.ORDER.DATE  is really
B.ORDER_DATE

and we remove that trailing comma and ignore the @ you query would look like this:
SELECT
      B.ORDER_DATE
    , B.ITEM_DESCRIPTION
    , B.QUANTITY
    , B.AMOUNT_BILLED
    , B.CANCEL_DATE
    , B.TYPE_OF_CANCEL
    , B.DAL_CANCELLED_BY
    , B.DAL_REASON_NOTES
    , B.DAL_REASON_TEXT
    , B.BILL_DATE

FROM BILLING.TRANSACTIONS AS B
      LEFT OUTER JOIN DAILY_ACTIVITY_LOG AS D
                  ON B.ID = CAST(D.BILLING_TRANSACTION_NUM AS varchar)

WHERE B.CANCEL_DATE = CURRENT_DATE
      AND B.CANCEL_DATE > CURRENT_DATE - 1

Open in new window

That FROM clause joins a billing transactions table to an activity log table using a join type that allows unmatched records in the transactions table to be listed; a LEFT OUTER JOIN. That is, IF there is no activity log information for a transaction, that transaction will still be shown.

A LEFT OUTER JOIN may also be used without the "outer" and are quite commonly referred to as "LEFT JOIN"

The JOIN CONDITION being used is "B.ID = CAST(D.BILLING_TRANSACTION_NUM AS varchar)"

This join condition isn't good from a performance perspective, as it changes data type and this robs the optimizer the opportunity of using an index on the field D.BILLING_TRANSACTION_NUM

The other issue with this join condition is it "looks weird" because B.ID would normally be associated with an integer (int or bigint) so it doesn't make much sense to change D.BILLING_TRANSACTION_NUM to varchar if it is being compared to an integer. Hence there may be another type conversion occurring on B.ID (an "implicit type conversion") which will further slow down the join's performance.

Regarding the where clause, I don't know what "CURRENT_DATE" is meant to be but in SQL Server you can get the current date via a system function getdate() (this has date plus time) and you can get the current date via
CAST(getdate() AS DATE)
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PortletPaulfreelancerCommented:
by the way, are you actually using Microsoft SQL Server?
if not, what is your database type please?
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garyrobbinsAuthor Commented:
Thank you for the thorough response it answered some other questions i had also.
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