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SQL: The Complete Reference
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Relationships among SQL2 join types
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I The full outer join contains all of the rows from the inner join, plus each unmatched row from TBL1, NULL-extended, plus each unmatched row from TBL2, NULL-extended. Roughly speaking, its query results are equal to the left outer join plus the right outer join. I The union join contains all of the rows of TBL1, NULL-extended, plus all of the rows of TBL2, NULL-extended. Roughly speaking, its query results are the full outer join minus the inner join.
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Multitable Joins in SQL2
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An important advantage of the SQL2 notation is that it allows very clear specification of three-table or four-table joins. To build these more complex joins, any of the join expressions shown in Figure 7-13 and described in the preceding sections can be enclosed in parentheses. The resulting join expression can itself be used in another join expression, as if it were a simple table. Just as SQL allows you to combine mathematical
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Multitable Queries (Joins)
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operations (+, , *, and /) with parentheses and build more complex expressions, the SQL2 standard allows you to build more complex join expressions in the same way. To illustrate multitable joins, assume that a new PARENTS table has been added to the database containing the GIRLS and BOYS example we have been using. The PARENTS table has three columns: CHILD TYPE PNAME Matches the NAME column in the GIRLS or BOYS table Specifies FATHER or MOTHER First name of the parent
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A row in the GIRLS or BOYS table can have two matching rows in the PARENTS table, one specifying a MOTHER and one a FATHER, or it can have only one of these rows, or it can have no matching rows if no data on the child s parents is available. The GIRLS, BOYS, and PARENTS tables together provide a rich set of data for some multitable join examples. For example, suppose you wanted to make a list of all of the girls, along with the names of their mothers and the names of the boys who live in the same city. Here is one query that produces the list:
SELECT FROM ON JOIN ON WHERE GIRLS.NAME, PNAME, BOYS.NAME ((GIRLS JOIN PARENTS PARENT.CHILD = NAME) BOYS (GIRLS.CITY = BOYS.CITY)) TYPE = "MOTHER"
Because both of these joins are inner joins, any girl who does not have a boy living in the same city or any girl who does not have a mother in the database will not show up in the query results. This may or may not be the desired result. To include those girls without a matching mother in the database, you would change the join between the GIRLS and the PARENTS table to a left outer join, like this:
SELECT FROM ON JOIN ON WHERE GIRLS.NAME, PNAME, BOYS.NAME ((GIRLS LEFT JOIN PARENTS PARENT.CHILD = NAME) BOYS (GIRLS.CITY = BOYS.CITY)) (TYPE = "MOTHER") OR (TYPE IS NULL)
This query will include all of the girl/boy pairs, regardless of whether the girls have a mother in the database, but it will still omit girls who do not live in a city with
SQL: The Complete Reference
any of the boys. To include these girls as well, the second join must also be converted to a left outer join:
SELECT FROM ON LEFT ON WHERE GIRLS.NAME, PNAME, BOYS.NAME ((GIRLS LEFT JOIN PARENTS PARENT.CHILD = NAME) JOIN BOYS (GIRLS.CITY = BOYS.CITY)) (TYPE = "MOTHER") OR (TYPE IS NULL)
Note that the NULL-extension of the GIRLS rows by the outer join with their mothers also creates some additional complication in the WHERE clause. The girls without matching mothers will generate rows with not only a NULL mother s name (PNAME) column but also a NULL value in the TYPE column. The simple selection criterion:
WHERE (TYPE = "MOTHER")
would generate an unknown result for these rows, and they will not be included in the query results. But the entire reason for using the left outer join was to make certain they were included! To solve this problem, the WHERE clause is expanded to also test for, and allow, rows where the parent type is NULL. As one final example, suppose you want to generate a girl/boy listing again, but this time you want to include the name of the boy s father and the girl s mother in the query results. This query requires a four-table join (BOYS, GIRLS, and two copies of the PARENTS table, one for joining to the boys information to get father names and one for joining to the girls information to obtain mother names). Again the potential for unmatched rows in the joins means there are several possible right answers to the query. Suppose, as before, that you want to include all girls and boys in the boy/girl pairing, even if the boy or girl does not have a matching row in the PARENTS table. You need to use outer joins for the (BOYS join PARENTS) and (GIRLS join PARENTS) parts of the query, but an inner join for the (BOYS join GIRLS) part of the query. This SQL2 query yields the desired results:
SELECT FROM ON JOIN ON USING GIRLS.NAME, MOTHERS.PNAME, BOYS.NAME, FATHERS.PNAME ((GIRLS LEFT JOIN PARENTS AS MOTHERS ((CHILD = GIRLS.NAME) AND (TYPE = "MOTHER"))) (BOYS LEFT JOIN PARENTS AS FATHERS ((CHILD = BOYS.NAME) AND (TYPE = "FATHER")))) (CITY)
This query solves the WHERE-clause test problem in a different way by moving the test for the TYPE of parent into the ON clause of the join specification. In this position, the test for appropriate TYPE of parent will be performed when the DBMS finds matching
7:
Multitable Queries (Joins)
columns to construct the join, before the NULL-extended rows are added to the outer join results. Because the PARENTS table is being used twice in the FROM clause, in two different roles, it s necessary to give it two different table aliases so that the correct names can be specified in the select list. As this example shows, even a four-join query like this one can become quite complex with the SQL2 syntax. However, despite the complexity, the SQL2 query does specify precisely the query that the DBMS is to carry out. There is no ambiguity about the order in which the tables are joined, or about which joins are inner or outer joins. Overall, the added capability is well worth the added complexity introduced by the extended SQL2 FROM clause. Although none of the query examples included in this section had WHERE or ORDER BY clauses, they can be freely used with the extended FROM clause in SQL2. The relationship among the clauses is simple and remains as described earlier in this chapter. The processing specified in the FROM clauses occurs first, including any joins or unions. The join criteria specified in a USING or ON clause are applied as a part of the particular join specification where they appear. When processing of the FROM class is complete, the resulting table is used to apply the selection criteria in the WHERE clause. Thus, the ON clause specifies search criteria that apply to specific joins; the WHERE clause specifies search criteria that apply to the entire table resulting from these joins.
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