SQL Let s Join Together

Transcription

1 Schaumburg SQL Let s Join Together Birgitta Hauser bha@toolmaker.de / Hauser@SSS-Software.de TOOLMAKER Advanced Efficiency GmbH Tel. (+49) Landsberg am Lech - Bayertor Seite 2 1

2 Agenda SELECT Statement Joining Tables and Views Inner Join Left/Right Outer Join Full Join Left/Right Exception Join Cross Join Self Join NULL Values Union / Intersect / Except Common Table Expression Seite 3 SELECT Statement Seite 4 2

3 SELECT Statement Basic SELECT Statement * = Select all Columns Basic SELECT Statement Filter Data List Columns explicitly Select Rows by adding WHERE Conditions Basic SELECT Statement Sort Data Sort Rows by using an ORDER BY clause Seite 5 SELECT Statement WITH Common Table Expressions (CTE) Full Select SELECT Statement SELECT Columns/Fields (incl.scalar/ud functions) From Files/Tables/Views/UDTFs incl. Joins Where Conditions Group By Clause Having Clause Start With Connect By (Release 7.1 PTF) Order By Clause (Release 6.1) Fetch First x Rows Only (Release 6.1) Merge several Sub-Selects using: UNION / EXCEPT / INTERSECT SELECT Columns/Fields (incl.scalar/ud functions From Files/Tables/Views/UDTFs incl. Joins Where Conditions Group By Clause Having Clause Start With Connect By (Release 7.1 PTF) Order By Clause (Release 6.1) Fetch First x Rows Only (Release 6.1) Sub-Select Sub-Select ORDER BY Clauses Fetch First x Rows, Limit/Offset, For Update Of, Optimize For x Rows Seite 6 3

4 Join Tables and Views Seite 7 Join Tables/Views Type Of Join (Inner) Join: Left (Outer) Join: Right (Outer) Join: Full (Outer) Join: (Left) Exception Join: Right Exception Join: Cross Join: Description Only rows that are found in both tables/views are returned All rows from the first (left) table/view are returned even if there is no match in the second table/view If there is no equivalent in the second table, NULL values will be returned All rows from the second (right) table/view are returned even if there is no match in the first table/view If there is no equivalent in the second table, NULL values will be returned All rows of both tables are returned independent if there is a match or not All rows from the first (left) table/view are returned where in the second (right) table/view no match is found All rows from the second (right) table/view are returned where in the first (left) table/view no match is found All rows of the first table/view are connected to all rows of the second table (cartesian product) Seite 8 4

5 Join Tables/Views Inner Join Inner Join: all rows that are available in both tables Join within the WHERE conditions Join within the FROM clause Seite 9 Join Tables/Views Inner Join Joining Multiple Tables/Views Joining multiple Files Specify the JOIN type and the next table/view Specify a single or multiple key Columns after ON Seite 10 5

6 Join Tables/Views - Self Join A table/view can be joined with itself Key fields can be joined by any of the following operators = (Equal) > (Greater) >= (Greater Equal) <> (Not Equal) < (Lower) <= (Lower Equal) Join year with the previous year Seite 11 Join Tables/Views - Self Join Running Totals Seite 12 6

7 NULL Values Seite 13 NULL Predicate What are NULL Values? Values out of the Valid Range Neither Blank nor Zero nor x 00 Selecting everything between high and low value will NOT return NULL values NULL Value in the Database Separate flag set to *On/*Off Must be checked separately SQL: RPG: Embedded SQL: using the NULL predicate IS NULL / IS NOT NULL (= not a NULL value) Built-In-Function: %NullInd() Indicator Variable Seite 14 7

8 Select identical Values Identical values Identical NULL values are not returned Identical NULL values are returned by testing for NULL values in both columns Seite 15 Distinct Predicate Compares a Value to another Value NULL values are considered as equal Contrary to comparing by using the Equal Sign (=) Seite 16 8

9 Distinct Predicate Identical values incl. NULL values returned Distinct Predicate in a Case Clause Seite 17 Scalar Functions Convert NULL Values COALESCE(Column, Default1, Default2, DefaultN) or IFNULL(Column, Default) Convert NULL Value into Default Value Available for all Data Types Convert NULL value into a Default value NULLIF(Column, ToReplace) Convert specified Value into NULL-Value Available for all data types Convert a Default value into a NULL value Seite 18 9

10 NULL Values and Aggregate Function NULL values are ignored by aggregate functions Important forcount(), AVG(), MIN(), MAX() Example: 6 rows with 2 NULL values Count(Column) = 4 COUNT(*) Counts ALL rows even if there is a row with only NULLs Positions without delivery quantity are considered Positions without delivery quantity are ignored Seite 19 Various Join Methods Seite 20 10

11 Join Tables/Views - Left / Right Outer Join Left Outer Join All rows from the 1st table/view All rows from the second table where a match is found Order headers without Order positions Right Outer Join All rows from the 2nd table/view All rows from the first table where a match is found Order positions without Order headers Seite 21 Join Tables/Views - Left/Right (Outer) Join Compare sales of the current year with the sales of the previous year (left outer join) Seite 22 11

12 Join Tables/Views - Left/Right (Outer) Join Compare sales of the current year with the sales of the next year (right outer join) Seite 23 Join Tables / Views Full Outer Join Coalesce: Convert NULL value into a Default Value Seite 24 12

13 Join Tables/Views - Exception Join Exception Join All rows from the left table without match in the second Table Predicate EXISTS Returns true if rows exist Seite 25 Join Tables/Views - Cross Join Cross Join cartesian product Join Tables/Views with a single or a few rows No columns for to join available Seite 26 13

14 Union, Intersect and Except Seite 27 SELECT Statement Full Select SELECT Columns/Fields (incl.scalar/ud functions) From Files/Tables/Views/UDTFs incl. Joins Where Conditions Group By Clause Having Clause SELECT Statement Sub-Select Merge several Sub-Selects using: UNION / EXCEPT / INTERSECT SELECT Columns/Fields (incl.scalar/ud functions From Files/Tables/Views/UDTFs incl. Joins Where Conditions Group By Clause Having Clause Sub-Select ORDER BY Clauses Fetch First x Rows, Limit/Offset, For Update Of, Optimize For x Rows Seite 28 14

15 UNION Clause A Union-Clause allows to combine several SELECT Statements and execute them as if it was a single one. Example: As soon as an order is completed it is copied to a history table. If a specific order is searched, both tables (OrderHeader and History Table) must be searched. Prerequisites for UNION Clauses Identical Number of Columns in the SELECT List Compatible data types in the selected columns Add ALL or DISTINCT controls the output of Duplicates UNION (DISTINCT): Returns No Duplicats DISTINCT is Default and optional Order is required Find duplicates Performance! UNION ALL: Returns Duplicates Seite 29 UNION - Clause Merge rows from multiple tables without duplicates Merge rows from multiple tables including duplicates Duplicates Seite 30 15

16 UNION Clause Example Order By for the complete result Seite 31 INTERSECT Clause Composition of several Sub-Selects Display all rows that are identical in both tables/views Prerequisites analog UNION Identical Number of Columns in the SELECT List Compatible data types in the selected columns Current Table CustNo Sales , , , , , ,00 Saved Table CustNo Sales , , , , ,00 Select * From Current Intersect Select * From Saved Result CustNo Sales , , ,00 Seite 32 16

17 EXCEPT Clause Composition of several Sub-Selects Display all rows that are in the 1st table/view but not in the 2nd one Prerequisites analog UNION Identical number of columns in the SELECT List Compatible data types in the selected columns Current Table CustNo Sales , , , , , ,00 Saved Table CustNo Sales , , , , ,00 Select * From Current Except Select * From Saved Result CustNo Sales , , ,00 Seite 33 INTERSECT Returns all rows that completely match in both tables Determine duplicates EXCEPT Check whether 2 tables are a complete match (all rows/ all columns) Check whether all rows and all columns in both tables are identical: No row returned Both tables are identical Seite 34 17

18 Common Table Expressions Seite 35 SELECT Statement WITH Common Table Expressions (CTE) Full Select SELECT Columns/Fields (incl.scalar/ud functions) From Files/Tables/Views/UDTFs incl. Joins Where Conditions Group By Clause Having Clause SELECT Statement Sub-Select Merge several Sub-Selects using: UNION / EXCEPT / INTERSECT SELECT Columns/Fields (incl.scalar/ud functions From Files/Tables/Views/UDTFs incl. Joins Where Conditions Group By Clause Having Clause Sub-Select ORDER BY Clauses Fetch First x Rows, Limit/Offset, For Update Of, Optimize For x Rows Seite 36 18

19 Common Table Expressions (CTE) Part of Select Statements Introduced with release V4R4M0 Must be specified before the Full-Select With CTE as (Select Pseudo-Views only valid for the current SELECT Statement Alternate for (nested) Sub-Selects Complex SQL-Queries can be split into smaller parts CTE can be used in other CTEs Order By and Fetch First X Rows only are allowed within CTEs Seite 37 Common Table Expressions Example CTE x: Accumulates the daily amounts on customer and year Selects only the amounts for the years 2008, 2009, 2010 CTE y: Accumulates the amount / year for the same period Based on the x CTE Final Select: Joins both CTEs on the SalesYear column Displays all columns from the x CTE and the TotalYear column and computes the percentage of the customer sales Seite 38 19

20 Common Table Expressions Example Seite 39 Common Table Expressions Example Calculate the difference between the accumulated sales of the customer of the current year with the accumulated sales of the customer of the previous year Seite 40 20

21 Any Questions Seite 41 Landsberg am Lech Seite 42 21

22 Special Thanks to Deon (Kit) von Blerk For proof-reading/correcting the English translation of these presentations Holger Scherer For providing an AS/400 (IBM i-system) enabling the creation of the samples/code used in these presentations Your data is save! in the bunker Seite 43 References IBM i information center SQL Reference IBM developer works IBM i Hierarchical Queries with Db2 Connect By Seite 44 22

23 Speaker s Biography Birgitta Hauser has been a Software and Database Engineer since 2008, focusing on RPG, SQL and Web development on IBM i at Toolmaker Advanced Efficiency GmbH in Germany. She graduated with a business economics diploma, and started programming on the AS/400 in She also works in consulting and education as a trainer for RPG and SQL developers. Since 2002 she has frequently spoken at the COMMON User Groups and other IBM i and Power Conferences in Germany, other European Countries, USA and Canada. In addition, she is co-author of two IBM Redbooks and also the author of several articles and papers focusing on RPG and SQL for a German publisher, ipro Developer and IBM DeveloperWorks. In 2015 she received the John Earl Speaker Scholarship Award. Seite 45 SQL - Joins? Yes i can! Tack så mycket! Birgitta Hauser bha@toolmaker.de Hauser@SSS-Software.de Seite 46 Data3 Höstkonferensen SQL - Let's Join Together Birgitta Hauser Toolmaker Advanced Efficiency GmbH Westendstrasse Kaufering Tel. (+49)