DEVELOPING DATABASE APPLICATIONS (INTERMEDIATE MICROSOFT ACCESS, X405.5)

Transcription

1 Technology & Information Management Instructor: Michael Kremer, Ph.D. Database Program: Microsoft Access Series DEVELOPING DATABASE APPLICATIONS (INTERMEDIATE MICROSOFT ACCESS, X405.5) Section 4

2 AGENDA 13. Advanced Query Design 14. Action Queries 15. SQL Sub Queries

3 Advanced Queries 13.

4 13.1 OVERVIEW OF QUERIES IN MS ACCESS 157 Microsoft Access contains many different types of queries. Categorize Queries within SQL language. The SQL language consists of four main parts: DQLData Query Language, the part that selects data and returns data in a specific format. No changes to data are made. Select Query Parameter Query Total Query Crosstab Query

5 13.1 OVERVIEW OF QUERIES IN MS ACCESS 158 DML Data Manipulation Language, the part that makes changes to data. Action Queries: Append Update Delete DDLData Definition Language, the part that builds a database, such as tables, indexes, and relationships. Action Queries: Make Table Query SQL Query DCL Data Control Language, the part that allows access to the database.

6 13.2 NULL OR NOT NULL 159 Relational databases must deal with the issue of nulls, or the absence of a value. MS Access is a powerful relational database system, and it allows the use of null to differentiate between two states of nothing : Null: Represents the absence of a value (currently) or not applicable. It may have a distinct value at a later time. Empty, Nothing: For text zero-length, for numbers 0: A distinct value of zero, that is, it is known to be nothing.

7 13.2 NULL OR NOT NULL 160 A zero-length string is two quotation marks without any space in between( ). Whether to allow null and/or zero-length values for a field can be set at the table level. Two properties for each field, the Required and Allow Zero Length property are available to specify the behavior. Now that we know the difference between null and zero-length strings, we can also construct query criteria to search for such records. To search for records that are not null, type in the criteria row for a field not null or is not null.

8 13.2 NULL OR NOT NULL 161

9 13.2 NULL OR NOT NULL 162

10 13.2 NULL OR NOT NULL 163

11 13.3 CALCULATIONS IN QUERIES 164 Queries are powerful due to the fact that not only database fields can be selected or analyzed, but additional fields in queries can be created based on string concatenation or mathematical calculation. Furthermore, derived or calculated expressions can be utilized as criteria in a query. Derived or calculated information can be used in three places in a query: Calculated or derived field Calculated or derived criteria Calculated or derived expressions to update data in update queries Furthermore, functions can be utilized in calculated or derived expressions. Some functions are part of SQL (Sum, Count), whereas other functions come with VBA, the programming environment in MS Access (IIf, DateAdd).

12 13.3 CALCULATIONS IN QUERIES 165

13 13.3 CALCULATIONS IN QUERIES 166 Immediate If (IIf) function, enables SQL to perform a multi-line If..Else..End if block. Every major database product has such kind of function or a similar function.

14 13.3 CALCULATIONS IN QUERIES 167

15 13.3 CALCULATIONS IN QUERIES 168

16 13.3 CALCULATIONS IN QUERIES 169 Calculated or derived expressions in Total queries require some special attention. An expression can be used in a total query in three different positions: Group By Field Aggregate Field Non-Aggregate Field Using an expression in an aggregate field requires some special handling: Total row value must be set to Expression. At least one aggregate function must be used in that expression.

17 13.3 CALCULATIONS IN QUERIES 170

18 13.4 THE EXPRESSION BUILDER 171 The Expression Builder is an amazing tool to build expressions and references to database objects. The expression builder can be launched in MS Access from these locations: Table design: Query design: Form design: Report design: Macro design: Default Value and Validation Rule Field Row, Criteria Row, Update To Row (Update queries only) Control Source, Default Value, Validation Rule, Event Properties Control Source, Event Properties Depending on actions, such as SetValue

19 13.4 THE EXPRESSION BUILDER Instructions and Help link: Get information about the context in which you are entering the expression. 2 Expression box: Type your expression here, or add expression elements by double-clicking items in the element lists below. 3 Expression Elements list: Click an element type to view its categories in the Expression Categories list

20 13.4 THE EXPRESSION BUILDER Expression Categories list: Click a category to view its values in the Expression Values list. If there are no values in the Expression Values list, double-click the category item to add it to the Expression box. 5 Expression Values list: Double-click a value to add it to the Expression box. 6 Help and information about the selected expression value: If available, click the link to see a Help article about the selected expression value.

21 13.4 THE EXPRESSION BUILDER 174 Add an element to an expression by using the element lists: 1. Click an item in the Expression Elements list. 2. If applicable, click a category in the middle list 3. Double-click an item in the right list. Intelli-Sense and Quick Tips New since Access 2010 List suggested options IntelliSense displays possible functions and other identifiers as you type an expression. 2 Quick Tips display a short description of the selected item. Accept a suggested value by pressing [Tab] or [Enter], use arrow keys to navigate through list.

22 13.4 THE EXPRESSION BUILDER 175 Quick Info and Help 1 2 Quick Info displays the 1 2 syntax of the function. Click the function name to open a Help topic about the function. Optional arguments are enclosed in square brackets. The argument that you are currently typing is displayed in bold text.

23 Action Queries 14.

24 14.1 MAKE-TABLE QUERY 176 The Make-Table query is a very useful query to move specific data over into a new table. It is also useful to create a subset of data for exporting purposes. A Make-Table query can include multiple tables, sorting, and criteria. There are further applications for a Make-Table query: To export data into different MS Access databases. To make a copy of data at a specific time during production, that is to preserve data. To make a backup copy of a table or tables. To move old records into an archive table. To create denormalized tables to improve performance for forms (readonly) or reports, or to create a data warehouse.

25 14.1 MAKE-TABLE QUERY 177

26 14.1 MAKE-TABLE QUERY 178

27 14.1 MAKE-TABLE QUERY 179 Make-Table query can also be used to make changes in the database design and move data into new tables. Horizontal design (phone/fax) does not allow for any new type of communication device. Create new table named tbl_communication to store any number for a given customer in a vertical manner. First use Make-Table query, select CustomerID, Phone, and derived field DeviceID: PH to create a new table.

28 14.1 MAKE-TABLE QUERY 180

29 14.1 MAKE-TABLE QUERY 181

30 14.2 APPEND QUERY 182 An append query copies records from one table or many tables and adds them to another existing table. New row is added to the QBE interface, the Append row. Specify for each column in the data source to which field data is to be appended. Be aware of the following possible problems: If the target table contains a primary key field, the records to be appended cannot have a null value nor a duplicate value for the primary key field. If the target table contains required fields, and/or the AllowZeroLength property is set to No, the records to be appended must adhere to these settings. If the target table contains validation rules or input masks, the data to be appended must conform accordingly. If the data type of a field in the source table differs from the target table, MS Access attempts to convert the data, if possible. If data type conversion is not possible, MS Access will set these fields to Null. If field size value differs between source and target, MS Access automatically truncates without warning.

31 14.2 APPEND QUERY 183 Error dialog message box when violating any target table rules: When selecting the same column more than once:

32 14.2 APPEND QUERY 184

33 14.2 APPEND QUERY 185

34 14.3 UPDATE QUERY 186 Update queries are used to make changes to a lot of data. Rather than updating data on a one by one basis, an update query performs the same in one single operation. Furthermore, using expressions, data can be updated based on conditions and circumstances. In the Update To row you can now build expressions using the Expression Builder tool, if needed. To update an existing field, use the field reference of the field to be updated in the expression and then add the calculation/expression.

35 14.3 UPDATE QUERY 187

36 14.3 UPDATE QUERY 188

37 14.4 DELETE QUERY 189 Delete query deletes all or some records from a table or tables. The table object is not deleted, just the data contained in the table. If no criteria are specified, all records are deleted, otherwise records affected by the criteria are deleted. Using a Delete query you can only delete entire records, not individual fields within a record. Two Options: FROM WHERE FROM is used to delete records from a table. WHERE is used to specify criteria.

38 14.4 DELETE QUERY 190

39 14.4 DELETE QUERY 191

40 SQL Sub Queries 15.

41 15.1 OVERVIEW OF SQL SUB QUERIES 192 SQL sub queries are basically embedded Select statements within a main SQL statement. First, the sub query is evaluated, and the result is used in the main query to retrieve data. Three type of sub queries, categorized by what they return: Scalar sub query: Returns one row and one column, that is, one value. Row sub query: Table sub query: Returns multiple rows, but one column. Returns multiple rows and columns. You can place a sub query in different positions inside a query: In SELECT clause as a field expression (scalar sub query) In FROM clause as a data source( table sub query) In WHERE clause as a filter expression (scalar, row or table sub query) In Update To or Delete rows (scalar sub query) In an append query as the source of data

42 15.2 SCALAR SUB QUERIES 193

43 15.2 SCALAR SUB QUERIES 194

44 15.3 TABLE SUB QUERIES 195

45 15.3 TABLE SUB QUERIES 196 Use the Any or Some keyword to retrieve records meeting at least one value in the list of values provided by the table sub query. Use the All keyword to display records that meet all values retrieved by the table sub query.

46 15.3 TABLE SUB QUERIES 197

47 15.3 TABLE SUB QUERIES 198 Exists keyword Returns true if rows in a sub query exist, otherwise it returns false.