The ANY/SOME/IN representations are all interpreted as a SOME operation. Both EXISTS and INTERSECT are parsed as an EXISTS with correlated subquery. The EXISTS and INTERSECT cases differ only in that the latter comes with an implicit DISTINCT (grouping on all projected columns). The two main workaround syntax groups are EXISTS/INTERSECT, and ANY/SOME/IN. The common workaround semi join syntaxes are transformed to a "real" internal semi join early in the query compilation process (well before even a trivial plan is considered). Early OptimizationsĪlthough T-SQL lacks direct SEMI JOIN syntax, the optimizer knows all about semi joins natively, and can manipulate them directly. Naturally, the final plan choice is still a cost-based decision among the explored alternatives. The transformation to inner join is explored early because the optimizer knows more tricks for inner equijoins than it does for semi joins, potentially leading to more optimization opportunities. The execution plan shows that the optimizer introduced an aggregate (grouping on INV.ProductID) to ensure that the inner join can only return Product rows once, or not at all (as required to preserve the semi join semantics): The AdventureWorks example below shows a semi join being removed entirely, due to a trusted foreign key relationship: Transformation and simplificationĪ logical semi join might be simplified away or replaced with something else during query compilation and optimization. Nevertheless, a logical semi join expressed in T-SQL might not lead to an execution plan using a row goal for several reasons, which we will unpack next. The description of a semi join above naturally hints at the application of a row goal, since we are interested in finding any matching row in B, not all such rows. T-SQL currently lacks support for direct syntax like FROM A SEMI JOIN B ON A.x = B.y, so we need to use indirect forms like EXISTS, SOME/ANY (including the equivalent shorthand IN for equality comparisons), and set INTERSECT. Regular join may return columns from either (or both) join inputs.Semi join is defined to only return columns from input A.Regular join duplicates rows if there are multiple matches on the join predicate.Semi join either returns each row from input A, or it does not.The essential differences between a semi join and a regular join are: This article will help you understand when, and why, a semi join invokes the optimizer's row goal logic.Ī semi join returns a row from one join input (A) if there is at least one matching row on the other join input (B). It is rather less commonly appreciated that semi joins (and anti joins) can introduce a row goal as well, though this is somewhat less likely than is the case for TOP, FAST, and SET ROWCOUNT. It is relatively well-known that using TOP or a FAST n query hint can set a row goal in an execution plan (see Setting and Identifying Row Goals in Execution Plans if you need a refresher on row goals and their causes). Part 1: Setting and Identifying Row Goals.Therefore, they’re used in the payment table as a foreign key.This post is part a series of articles about row goals. Note that the student_id and course_code columns form a primary key in the enrollment table. In the next condition, we get the course_code column from the enrollment table and course_code from the payment table. In the first part, we use the student_id column from the enrollment table and student_id from the payment table. P.course_code=e.course_code AND p.student_id=e.student_id How can we join the tables with these compound keys?Įasy! We just need to use a JOIN clause with more than one condition by using the AND operator after the first condition. In the second table ( payment), we have columns that are a foreign compound key ( student_id and course_code). In one joined table (in our example, enrollment), we have a primary key built from two columns ( student_id and course_code). If you’d like to get data stored in tables joined by a compound key that’s a primary key in one table and a foreign key in another table, simply use a join condition on multiple columns. Let’s show each student’s name, course code, and payment status and amount. The payment table has data in the following columns: foreign key ( student_id and course_code, the primary keys of the enrollment table), status, and amount. The enrollment table has data in the following columns: primary key ( student_id and course_code), is_active, and start_date. The student table has data in the following columns: id (primary key), first_name, and last_name. Our database has three tables named student, enrollment, and payment. You want to join tables on multiple columns by using a primary compound key in one table and a foreign compound key in another.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |