DB Implementation: MS Access Queries

MS Access Queries Database Queries Core DBA skill From SQL to Query by Example (QBE) What does it do? Find target information Retrieve, Filter, Sort, Aggregate/Summarize Manipulate data Perform calculations Add, Change, Delete, Combine data in tables Assemble/Supply data for forms and reports How does it work? Access translates QBE to SQL SQL performs data manipulations based on Relational Algebra Access queries create a dynaset (“live” view of table) changes made in data by query is reflected in underlying tables Database System2

Access Queries: Views Datasheet view For displaying the result of the query Useful for reviewing/validating the query Design View For creating/modifying a query using drag & drop GUI (i.e., QBE) Consists of Diagram Pane & Grid Pane Add tables/queries to the Diagram Pane Add fields to the Grid Pane (Field row) Can sort/filter/compute by fields Sort row: set to Ascending/Descending Criteria row: use Expression to apply data filter Total row: compute (sum, min, max, count, etc.) of each field Automatically generates SQL statements SQL View For creating/modifying a query by manually writing SQL statements Only way to create SQL-specific queries Union/Data-definition/Pass-through query Database System3

Access Queries: Basic Types Simple Query Uses one table/query To generate a subset (row/column) of a table Multi-table Query Joins multiple tables/queries To merge small chunks of data in normalized tables Linked tables are automatically linked in in the Query Design Grid Creating a link in the Query Design Grid does not permanently link tables Select Query Selects records that meet given criteria Does not change the data Parameter Query Prompts for query criteria values (parameters) to run a dynamic query Database System4

Access Queries: Sort & Filter Datasheet View Sorting Click column and right-click Filtering Filter Tool (Home tab) – Filter by Selection – Filter by Form – Advanced Filter/Sort Design View Sort using the Sort Row Sort priority is from left to right for multiple sort Filter using the Criteria Row Criteria in multiple rows make OR query Criteria in single row make AND query Criteria in a single cell – AND/OR – Wildcards (*) in Like and Between » e.g. Like “A*”, Like “[A-C]*”, Like “*av*”, Like “ave?”, Not Like “A*” » e.g. Between 1950 and 1960, >1960 Database System5

Access Queries: Query Criteria Query Criteria Expressions Database System6 ExpressionReturns Between #1/1/99# and #12/31/99# Dates from 1/1/99 to 12/31/99 In (“John”,”Mary”,”James”)Records with John, Mary, or James Is NullRecords with no entry Like “Acc?ss”‘Acc’ followed by any character, followed by ‘ss’ Like “*s”Ends in ‘s’ Like “v*”Starts with ‘v’ <1000Less than Equal to 1000 Like “[A-C]??”Starts with A through C and has two more characters ????Any four characters Len([Surname])= Val(4)Any Surname of 4 characters Right([Surname],2)= “ss”Any Surname ending in “ss” Left([Surname],2)= “ac”Any Surname starting with “ac”

Access Queries: Expressions Using expressions to create a calculated column Enter expressions in blank column of query design view NAME: [Field1] operator [Field2] Format the display in Format property of the field Examples Simple math DiscountPrice: [Discount] * [StandardPrice] String Concatenation Name: [FirstName] & “ ” & [Lastname] Date & Time math DateDiff(Interval, BeginDate, EndDate) – HireAge: DateDiff(“yyyy”,[BirthDate],[HireDate]) DateAdd(Interval, Number, Date) – RetireDate: DateAdd(“yyyy”,25,[HireDate]) Customized Sorting Switch(expr1, value1, expr2, value2, etc.) – Switch([City]=“Seatle”, 1, [City]=“Redmond”, 2, etc.) Database System7

Access Queries: Dynamic Criteria Parameterized Queries Dynamic query based on varying criteria value e.g. retrieve books written by a given author Enter the parameter name in square brackets in criteria Do not use existing field names Can use expressions – e.g. Like "*" & [Last Name] & "*“ Query Criteria from Form Entries Use a form to enter query parameter values good for multiple parameter entries 1.Create a parameter query 2.Create a form with unbound input control for each of query parameters 3.Add a command button that will run the parameter query Run Query Action of Miscellaneous Category 4.Reference the form controls that hold parameter values from the query i.e. change the criteria to form controls Parameter name = fully qualified name of form control Database System8

Access Queries: Joins Inner Join Default join in Access (i.e. Natural Join) Returns only the records where joined fields are equal in both tables Left Outer join Returns all records from the left table Right Outer join Returns all records from the right table Database System9