free barcode generator using vb.net Figure 7-14: Relationships among SQL2 join types in Software

Create QR Code in Software Figure 7-14: Relationships among SQL2 join types

Figure 7-14: Relationships among SQL2 join types
QR Code JIS X 0510 Decoder In None
Using Barcode Control SDK for Software Control to generate, create, read, scan barcode image in Software applications.
Draw QR Code In None
Using Barcode maker for Software Control to generate, create QR-Code image in Software applications.
Finally, it's useful to examine the relationships between the sets of rows produced by the cross join, the various types of outer joins, and the inner join shown in Figure 7-14. When joining two tables, TBL1 with m rows and TBL2 with n rows, the figure shows that:
Scan QR Code In None
Using Barcode decoder for Software Control to read, scan read, scan image in Software applications.
QR Creation In C#.NET
Using Barcode generator for .NET framework Control to generate, create QR Code JIS X 0510 image in Visual Studio .NET applications.
- 132 -
QR Code Encoder In Visual Studio .NET
Using Barcode encoder for ASP.NET Control to generate, create QR-Code image in ASP.NET applications.
Print QR-Code In VS .NET
Using Barcode generator for Visual Studio .NET Control to generate, create QR Code image in .NET framework applications.
The cross join will contain m x n rows, consisting of all possible row pairs from the two tables. TBL1 INNER JOIN TBL2 will contain some number of rows, r, which is less than m x n. The inner join is strictly a subset of the cross join. It is formed by eliminating those rows from the cross join that do not satisfy the matching condition for the inner join. The left outer join contains all of the rows from the inner join, plus each unmatched row from TBL1, NULL-extended. The right outer join also contains all of the rows from the inner join, plus each unmatched row from TBL2, NULL-extended. 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. 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.
Paint QR Code In VB.NET
Using Barcode drawer for VS .NET Control to generate, create QR Code JIS X 0510 image in VS .NET applications.
Code 128C Printer In None
Using Barcode generator for Software Control to generate, create ANSI/AIM Code 128 image in Software applications.
Multi-Table Joins in SQL2
Generate Bar Code In None
Using Barcode generation for Software Control to generate, create barcode image in Software applications.
Painting ECC200 In None
Using Barcode encoder for Software Control to generate, create Data Matrix image in Software applications.
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 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 multi-table 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 GIRLS or BOYS table Specifies FATHER or MOTHER First name of the parent
GS1 - 13 Printer In None
Using Barcode drawer for Software Control to generate, create EAN 13 image in Software applications.
Drawing Barcode In None
Using Barcode printer for Software Control to generate, create barcode image in Software applications.
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 multi-table 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 GIRLS.NAME, PNAME, BOYS.NAME ((GIRLS JOIN PARENTS PARENT.CHILD = NAME) (BOYS
4-State Customer Barcode Maker In None
Using Barcode maker for Software Control to generate, create USPS OneCode Solution Barcode image in Software applications.
GTIN - 13 Creation In Objective-C
Using Barcode maker for iPhone Control to generate, create GTIN - 13 image in iPhone applications.
- 133 -
Create UCC - 12 In Objective-C
Using Barcode maker for iPad Control to generate, create UPC Symbol image in iPad applications.
Encoding ECC200 In None
Using Barcode generation for Online Control to generate, create ECC200 image in Online applications.
ON (GIRLS.CITY = BOYS.CITY)) WHERE 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 GIRLS.NAME, PNAME, BOYS.NAME FROM ((GIRLS LEFT JOIN PARENTS ON PARENT.CHILD = NAME) JOIN (BOYS ON (GIRLS.CITY = BOYS.CITY)) WHERE (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 any of the boys. To include these girls as well, the second join must also be converted to a left outer join: SELECT GIRLS.NAME, PNAME, BOYS.NAME FROM ((GIRLS LEFT JOIN PARENTS ON PARENT.CHILD = NAME) LEFT JOIN (BOYS ON (GIRLS.CITY = BOYS.CITY)) WHERE (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 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")))
Encoding Bar Code In Objective-C
Using Barcode creation for iPad Control to generate, create barcode image in iPad applications.
Recognizing Data Matrix 2d Barcode In Java
Using Barcode scanner for Java Control to read, scan read, scan image in Java applications.
- 134 -
Encode Code 128 Code Set C In Java
Using Barcode drawer for BIRT reports Control to generate, create ANSI/AIM Code 128 image in Eclipse BIRT applications.
Making DataMatrix In Objective-C
Using Barcode creator for iPhone Control to generate, create DataMatrix image in iPhone applications.
USING (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 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.
Summary
This chapter described how SQL handles queries that combine data from two or more tables: In a multi-table query (a join), the tables containing the data are named in the FROM clause. Each row of query results is a combination of data from a single row in each of the tables, and it is the only row that draws its data from that particular combination. The most common multi-table queries use the parent/child relationships created by primary keys and foreign keys. In general, joins can be built by comparing any pair(s) of columns from the two joined tables, using either a test for equality or any other comparison test. A join can be thought of as the product of two tables from which some of the rows have been removed. A table can be joined to itself; self-joins require the use of a table alias. Outer joins extend the standard (inner) join by retaining unmatched rows of one or both of the joined tables in the query results, and using NULL values for data from the other table. The SQL2 standard provides comprehensive support for inner and outer joins, and for combining the results of joins with other multi-table operations such as unions, intersections, and differences.
- 135 -
Copyright © OnBarcode.com . All rights reserved.