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You cannot play with the ON clause's order any way you'd like. There's a certain relationship that must be maintained between the order of the specified tables and the order of the specified ON clauses for the query to be valid. The relationship is called a chiastic relationship. A chiastic relationship is neither unique to SQL nor to computer science; rather, it appears in many fields, including poetry, linguistics, mathematics, and others. In an ordered series of items, this relationship correlates the first item with the last, the second with the next to last, and so on. For example, the word ABBA has a chiastic relationship between the letters. As an example for a chiastic relationship in mathematics, recall the arithmetic sequence I described in the last chapter: 1, 2, 3, ..., n. To calculate the sum of the elements, you make n/2 pairs based on a chiastic relationship (1 + n, 2 + n 1, 3 + n 2, and so on). The sum of each pair is always 1 + n; therefore, the total sum of the arithmetic sequence is (1 + n) * n / 2 = (n + n2) / 2. Similarly, the relationship between the tables specified in the FROM clause and the ON clauses must be chiastic for the query to be valid. That is, the first ON clause can refer only to the immediate two tables right above it. The second ON clause can refer to the previously referenced tables and to an additional one right above them, and so on. Figure 5-5 illustrates the chiastic relationship maintained in the last query. The code in the figure was slightly rearranged for readability.
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Figure 5-5. Chiastic relationship in a multi-join query
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Without using parentheses, the queries are not very readable and you need to be aware of the chiastic relationship in order to write a valid query. Conversely, if you do use parentheses, the queries are more readable and intuitive and you don't need to concern yourself with chiastic relationships, as parentheses force you to write correctly.
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Semi joins are joins that return rows from one table based on the existence of related rows in the other table. If you return attributes from the left table, the join is called a left semi join. If you return attributes from the right table, it's called a right semi join. There are several ways to achieve a semi join: using inner joins, subqueries, and set operations (which I'll demonstrate later in the chapter). Using an inner join, you select attributes from only one of the tables and apply DISTINCT. For example, the following query returns customers from Spain that made orders: SELECT DISTINCT C.CustomerID, C.CompanyName FROM dbo.Customers AS C JOIN dbo.Orders AS O ON O.CustomerID = C.CustomerID WHERE Country = N'Spain';
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You can also use the EXISTS predicate as follows: SELECT CustomerID, CompanyName FROM dbo.Customers AS C WHERE Country = N'Spain' AND EXISTS (SELECT * FROM dbo.Orders AS O WHERE O.CustomerID = C.CustomerID);
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If you're wondering whether there's any performance difference between the two, in this case the optimizer generates an identical plan for both. This plan is shown in Figure 5-6.
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Figure 5-6. Execution plan for a left semi join
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