sql level 2 - powerpoint joins

8/11/2019 SQL level 2 - Powerpoint Joins

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Overview Natural Joins

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Joins are a technique for accessinginformation across tables.

Conceptually, a jo n takes the columns fromtwo tables and creates a new virtual table

with all of the columns. This is in contrast to a un on, which takes the

rows from two tables and forms a new tablewith all of the rows.

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SELECT column_list FROM table_list

SELECT

sid, name, majr,mcode, mname, dcode,dcode, dname

FROM

students, majors, divisions

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How do we get the right columns to line up? That is, how do we insure that the rows in the

students table match up with the right rowsin the majors table, match up with the rightrows in the division table?

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Via the WHERE Clause we can specify whichrows are to be included/maintained in theresult table.

WHEREstudents.majr = majors.mcode

and majors.dcode = divisions.dcode

How does this work?

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Conceptually, the FROM clause creates theCartesian product, or the cross product, ofthe two (or more) named tables.

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The WHERE Clause then selects which rowsare to be maintained in the result table.

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As the diagram / model suggests, joiningtables does not effect base tables.

Joining tables does not create base tables.

Joining tables creates working tables orvirtual tables that exist only for the durationof the inquiry.

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Another name for the cross product orCartesian product of two tables.

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Is a join based on equality, uses the equalityoperator (=).

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A form of the equi join, wherein theduplicated column is not displayed in theresult table.

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Is a join based on any comparison operator,other than the equality operator.

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Any kind of join based on any of thecomparison operators.

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These examples have all been inner joins. Aninner join is bound by a search expression,and disregards any rows where the searchcondition is not met.

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SELECT column_list FROM table_1CROSS JOIN table_2

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An outer join insures that rows from eitherone, or both tables are presented in the finalresult table.

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Specifies that each row in the first table (aslisted in the from clause) will be included inthe final result table.

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SELECT column_list FROM table_1 AS t1, table_2 AS t2

WHERE t1.column = t2.column

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WHERE t1.column = t2.column(+)

Note the plus sign goes with the table forwhich null values should be generated

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SELECT column_list FROM table_1LEFT OUTER JOIN table_2

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Specifies that each row in the second table(as listed in the from clause) will be includedin the final result table.

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WHERE t1.column(+) = t2.column

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Specifies that each row in each table (asmentioned in the from clause) will beincluded in the final result table.

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WHERE t1.column(+) = t2.column(+)

Unfortunately this doesnt work!

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SELECT column_list FROM table_1FULL OUTER JOIN table_2

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sql level 2 - powerpoint joins

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