Advanced Topics in Access: Queries

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1 I. Types of Queries A. Select: displays selected data from one or more tables B. Delete: removes records from a table based on criteria specified C. Make Table: creates a new table from existing table(s) and populates with qualified records D. Append: appends qualified records to an existing table based on specified criteria E. Update: globally changes records with a qualified field value to another specified value F. Crosstab: creates a two-dimensional grid of summary data from one or more tables Open the CemSurvey database and look at the Cemeteries table. II. Terminology A. Query By Example (QBE) Window 1. Field List 2. Design Grid 3. Field Header 4. Table 5. Sort 6. Show 7. Criteria III. Selection Queries 8. Datasheet View/Design View/SQL View button 9. Run Query ( Bang ) button A. Specifying Fields for Inclusion in Query

2 1. Drag-and-drop field from the field list box to a column of the design grid 2. Double click a field in the field list box 3. Use the drop box in the field row of the design grid to select a field 4. Shortcut: To specify several fields at once, use the shift or control key in the list box and drag all to the design grid. 5. Shortcut: To specify all fields in a table, just drag the asterisk ( * ) item from the field list box. B. Specifying Criteria 1. Enter a valid expression in the appropriate column of the query design grid. 2. Entering expressions for different fields in the same row means the criteria are joined by a logical AND (for example, Color = red AND Size = small ). 3. Entering expressions for different fields in separate rows means the criteria are joined by a logical OR (for example, Type = SUV OR DriveTrain = 4WD ) 4. Complex combinations of logical ANDs and logical ORs may be formed (for example, Type = SUV AND Mfr = Ford OR Mfr = Volvo ) 5. A parameter query is useful when the criteria value may change from run to run. Embed a question in square brackets in the criteria row under the appropriate column. The user s response is taken for the criteria value at run time ([Which manufacturer would you like?]). 6. Fields used as criteria in the query do not necessarily have to appear in the query results (simply clear the Show box). Devise a query to return all cemetery plots in Kentucky. mwstate Modify it to select only those in Knott County, Kentucky. mwcounty Now get all plots in Knott or Pike County, Kentucky (careful!). mwpike Write a parameter query to return plots in a state to be specified at run time. mwstateparameter C. Setting Sort Keys 2

3 1. In the appropriate column of the query design grid, choose Ascending or Descending in the Sort row. 2. You may select multiple sort keys, but you must arrange the columns so that the primary sort field occurs first in the grid (reading left-to-right), the secondary sort field is next, and so on for as many sort keys as you may have. D. Returning Query Results 1. Methods for executing a query a) From design mode: (1) Click the Run ( Bang ) button. (2) Click the Query View button. b) From the table of contents window (1) Double click the query (2) Select the query, click the open button 2. Recordset types a) Dynaset (default) recordsets are live. Any edits done to the query results recordset are transmitted back to the original table(s). Use the State parameter query to get all plots in Ohio ( OH ). Change the state to KY. Re-issue the query. b) Snapshot recordsets are unbound and edits are not permitted. Change the Recordset type property of the preceding query to Snapshot. Attempts to edit a field should fail. 3. Top Values a) Set the Top Values property to return just the first x number of records or x% number of records. b) Specify percents with a % sign (e.g. 10%) and absolute numbers with a simple number (e.g. 10). c) Works with the sort field settings to return top or bottom values. 4. Unique Values / Records 3

4 a) Setting the Unique Values property to Yes discards records that have duplicate values for fields in the datasheet view. b) Setting the Unique Records property to Yes discards records that have duplicate values in all fields of the underlying table(s). c) Either Unique Values or Unique Records may be set on, but both cannot be on at the same time. Get a list of all agencies who registered for workshops. Toggle unique values and unique records to show difference. mwunique 5. Aliasing Field Names a) Aliasing gives a field a new name in the query result. b) The alias precedes the field name in the field column (e.g. Car_Make: Make would rename the Make field to Car_Make in the query results). c) Used most often in Access query expressions (next section). IV. Expressions in Access Construct a query that returns all plots in Kentucky with a surname of Smith. Only return a Name field the concatenates First name and Surname (e.g. Robert Smith ) and the County name. mwsmiths A. An Access expression has a least one operator and at least one literal, identifier, or function. 1. Operators a) Arithmetic (1) +, -, *, / -- need no explanation (right?) (2) ^ -- exponentiation (3^2=9) (3) \ -- integer division, remainders are discarded ( 10 \ 3 = 3 ) (4) mod -- modulus or remainder division (10 mod 3 = 1 ) b) Assignment and comparison (1) = -- assignment to a variable or field (Make = Isuzu ) (2) compares two values and returns either True or False (a) < -- less than ( 3 < 5 returns TRUE ) (b) <= -- less than or equal to ( 3 <= 3 returns TRUE ) (c) = -- equal to ( 3 = 5 returns FALSE ) (d) >= -- greater than or equal to ( 3 >= 5 returns FALSE) 4

5 (e) > -- greater than ( 3 > 3 returns FALSE ) (f) <> -- not equal to ( 3 <> 3 returns FALSE ) c) Logical (or Boolean) (1) Combine logical expressions of TRUE, FALSE, or NULL (2) AND -- returns TRUE only if both expressions are TRUE (3) OR -- returns TRUE if either expression is TRUE (4) NOT -- negates a logical value (e.g. NOT TRUE returns FALSE) d) String (1) & -- preferred concatenation symbol ( Dodge & & Viper evaluates to Dodge Viper (2) + -- also concatenates as &, but should be reserved for addition to avoid ambiguity e) Identifier (1)! (bang) -- combines object classes and names to specify a single instance (e.g. Forms![AutomobileSales] specifies the form AutomobileSales as a member of the Forms class) (2). (period or dot) -- separates object names from object properties or methods (e.g. TextBox1.Value = Miata ) (3) To identify a specific field value on a form: Forms!MyForm![MyControl].Value f) Other (1) Is -- only used with Null to test whether or not value Is Null or Not Is Null (2) Like -- used with wildcard characters (* and?) to test string values (e.g. Like V* returns true for Volvo and Volkswagen, but not for Chevrolet) (3) Between -- used to test if a value lies within a range (e.g. Between 30 And 60 returns TRUE when tested with a value of 30, 45, or 60). Note that range limits are inclusive. (4) In -- used to see if a string value is in a list (e.g. In ( Compact, Subcompact, Economy ) 2. Literals a) Numeric b) Text -- enclosed in quote marks (e.g. Literal text ) c) Date/time -- enclosed in pound signs (#) such as #11/15/2001# 3. Identifiers a) Uniquely define an object b) syntax follows the form : class!collection!objectname (e.g. Forms!Students![First Name]) 5

6 c) note that any identifier with embedded blanks must be enclosed in square brackets (e.g. [First Name]) 4. Functions a) Functions evaluate to a single value when executed (e.g. Sum(1,2,3) returns 6 b) Usually take inputs in the form of a parameter list enclosed in parentheses after the function name c) There are over one hundred individual functions in Access in several categories (1) Date and time (2) Text manipulation (3) Conversion (numeric data to string conversion for example) (4) Math (5) Financial (6) Other d) search help system for details ( functions reference ) B. Expressions in Query Criteria 1. Enter expression on criteria row a) Like J* would return all records with field value beginning with J Get all Access classes (Like *Access*). mwlike1 Get all classes on teen dates (Like 11/1?/01 ) mwlike2 b) Between 5 And 10 would return all records with field values greater than or equal to 5 and less than or equal to 10 Get all classes between 11/8/01 and 11/13/01. mwbetween1 Get all student with last names C-G. mwbetween2, mwbetween2a c) >=Date()+1 would return all records with a date values of tomorrow or later Show all classes schedule before yesterday (< Date()-1) mwfunction1 d) In ( CA, NV, OR, WA ) would return records with State field values of these 4 states Show classes with codes of AA1, L1, or O1 mwin 2. Expressions in Calculated Fields in Queries 6

7 a) In the field row of the query grid, define a new field name followed by colon, then expression b) example -- given fields Quantity and Price, create the calculated field of extended price as ExtendedPrice: [Quantity] * [Price] Create a calculated field, Open, that subtracts spots reserved for SANDAG staff from capacity of class. Open: [Capacity] [SANDAG] mwcalc1 3. Using Groups in Query Expressions a) to create queries that aggregate data, use the sum button on the query button bar b) a new row, Total, is placed in the query grid c) twelve options are available in the Total row (1) Group By -- groups on like values, left to right across grid Create a list of all class descriptions. mwgroup1 Create a list of Instructors and their classes (add Instructor group before Description group). mwgroup2 (2) Sum -- totals numbers (use w/group By for subtotals) How many students can we accommodate? mwsum1 How many by each class? mwsum2 (save this query for later) (3) Avg (average) -- finds average for a group of numbers What is the average class capacity? mwaverage (4) Min (minimum) -- finds smallest value in a set of records When is does the first class take place? mwmin (5) Max (maximum) -- finds largest value in a set of records When is does the last class take place? mwmin (6) Count -- counts number of records How many classes is each instructor assigned to? mwcount1 Are there any students with same first name? mwcount2 (7) StDev (standard deviation) -- a measure of central tendency (8) Var (variance) -- a measure of central tendency (9) First -- finds first item in a group (best used with sorting) (10) Last -- finds last item in a group (best used with sorting) (11) Expression -- create a calculated field that includes an aggregate function in its expression How many students per class can we overbook if we assume 13% no show rate? mwsumexpression 7

8 V. Multitable Queries A. Inner (or Equi-) joins (12) Where -- used to set criteria for a field not defined in the groupings Get the aggregate capacity of all classes taught by M. Woodall mwsum2where d) note that the aggregate functions (2-8) don t count Null values 1. Returns records in result only when values of join fields in both tables are identical. 2. By far the most common type of join in Access. Retrieve names of everyone on the wait list for Intermediate Access. mwequi1 Get a count of students by agency. mwequi2 Get a count of registrations by agency. mwequi3 B. Outer joins 1. Returns records in result for all records in one table of the join and records from second table where join field values match. How many requests for each class? (normal, inner join). mwouter1 What about the Any Date classes? Use a left outer join for them. mwouter2 C. Self joins 1. Relate fields from a table to a copy of the same table. Who requested an Intermediate Access class? mwselfjoin1 Who requested both Intermediate AND Advanced Access? mwselfjoin2 D. Theta joins 1. Not equal (< >) joins. Query design window doesn t show these joins with lines, but rather through WHERE criteria. Are there any students registered twice? mwtheta VI. Crosstab Queries A. Used to present summary data in a spreadsheet-like format. B. Very useful when creating charts and graphs. 8

9 C. A Wizard is available to help with creating Crosstab queries, BUT it can only handle single tables. Create a single table Crosstab using a Wizard that shows the number of students by status in each of the classes. mwcrosstab1 Create a similar Crosstab, but add links to Class table so we can see the Description field (as a Row Heading) and add a link to the Status table so we can see the status labels (as Column Headings). mwcrosstab2 9

10 VII. Extras A. How many registrations in each status type (confirmed, flexible, wait, dropped)? B. 10