Visual Practice HOW TO

User Author: Jay Patterson

...SQL Command Processor Syntax

The SQL Command Processor is located under the Reports button on the toolbar.  The SQL Command Processor is used to manipulate data within tables in Visual Practice at the program level.  This is a very powerful feature which if used improperly can damage your data.  It is recommended that you do not use this feature unless you are instructed to do so by a technical support analyst.

The SQL following commands can be used in the SQL Processor window to display or manipulate tables, fields and data.  If you are familiar with SQL, these commands are SQL-92 compliant, with additional functionality as found in Microsoft Visual FoxPro 6.0

DELETE - SQL Command

Marks records for deletion.

Syntax

DELETE FROM [DatabaseName!]TableName
  [WHERE FilterCondition1 [AND | OR FilterCondition2 ...]]

Arguments

FROM [DatabaseName!]TableName

Specifies the table in which records are marked for deletion.

DatabaseName! specifies the name of a non-current database containing the table. You must include the name of a database containing the table if the database is not the current database. Include the exclamation point (!) delimiter after the database name and before the table name.

WHERE FilterCondition1 [AND | OR FilterCondition2 ...]

Specifies that Visual Practice marks only certain records for deletion.

FilterCondition specifies the criteria that records must meet to be marked for deletion. You can include as many filter conditions as you like, connecting them with the AND or OR operator. You can also use the NOT operator to reverse the value of a logical expression, or use EMPTY( ) to check for an empty field.

Remarks

Records marked for deletion aren't physically removed from the table until PACK is issued. Records marked for deletion can be recalled (unmarked) with RECALL.

If SET DELETED is set to ON, records marked for deletion are ignored by all commands that include a scope.

Unlike DELETE, DELETE - SQL uses record locking when marking multiple records for deletion in tables opened for shared access. This reduces record contention in multiuser situations, but may reduce performance. For maximum performance, open the table for exclusive use or use FLOCK( ) to lock the table.

UPDATE - SQL Command

Updates records in a table with new values.

Syntax

UPDATE [DatabaseName1!]TableName1
SET Column_Name1 = eExpression1
  [, Column_Name2 = eExpression2 ...]
  WHERE FilterCondition1 [AND | OR FilterCondition2 ...]]

Arguments

[DatabaseName1!]TableName1

Specifies the table in which records are updated with new values.

DatabaseName1! specifies the name of a non-current database containing the table. You must include the name of the database containing the table if the database is not the current one. Include the exclamation point (!) delimiter after the database name and before the table name.

SET Column_Name1 = eExpression1
  [, Column_Name2 = eExpression2

Specifies the columns that are updated and their new values. If you omit the WHERE clause, every row in the column is updated with the same value.

WHERE FilterCondition1 [AND | OR FilterCondition2 ...]]

Specifies the records that are updated with new values.

FilterCondition specifies the criteria that records must meet to be updated with new values. You can include as many filter conditions as you like, connecting them with the AND or OR operator. You can also use the NOT operator to reverse the value of a logical expression, or use EMPTY( ) to check for an empty field.

Remarks

UPDATE - SQL can only update records in a single table. Note that subqueries are supported in UPDATE – SQL.

Unlike REPLACE, UPDATE - SQL uses record locking when updating multiple records in a table opened for shared access. This reduces record contention in multiuser situations, but may reduce performance. For maximum performance, open the table for exclusive use or use FLOCK( ) to lock the table.

INSERT – SQL Command

Appends a record to the end of a table that contains the specified field values.

Syntax

INSERT INTO dbf_name [(fname1 [, fname2, ...])]
  VALUES (eExpression1 [, eExpression2, ...])

-or-

INSERT INTO dbf_name FROM ARRAY ArrayName | FROM MEMVAR

Arguments

INSERT INTO dbf_name

Specifies the name of the table to which the new record is appended. dbf_name can include a path and can be a name expression.

If the table you specify isn't open, it is opened exclusively in a new work area and the new record is appended to the table. The new work area isn't selected; the current work area remains selected.

If the table you specify is open, INSERT appends the new record to the table. If the table is open in a work area other than the current work area, it isn't selected after the record is appended; the current work area remains selected.

[(fname1 [, fname2 [, ...]])]

Specifies the names of the fields in the new record into which the values are inserted.

VALUES (eExpression1 [, eExpression2 [, ...]])

Specifies the field values inserted into the new record. If you omit the field names, you must specify the field values in the order defined by the table structure. If SET NULL is ON, INSERT – SQL attempts to insert null values into any fields not specified in the VALUES clause.

FROM ARRAY ArrayName

Specifies the array whose data is inserted into the new record. The contents of the elements of the array, starting with the first element, are inserted into the corresponding fields of the record. The contents of the first array element are inserted into the first field of the new record; the contents of the second array element are inserted into the second field, and so on.

Any default values for fields are ignored when you include the FROM ARRAY clause.

FROM MEMVAR

Specifies that the contents of variables are inserted into fields with the same names as the variables. If a variable doesn't exist with the same name as the field, the field is left empty.

Remarks

The new record contains the data listed in the VALUES clause or contained in the specified array or variables. The record pointer is positioned on the new record.

SELECT - SQL Command

Retrieves data from one or more tables.

Syntax

SELECT [ALL | DISTINCT] [TOP nExpr [PERCENT]]
  [Alias.] Select_Item [AS Column_Name]
  [, [Alias.] Select_Item [AS Column_Name] ...]

FROM [FORCE]
[DatabaseName!]Table [[AS] Local_Alias]
  [[INNER | LEFT [OUTER] | RIGHT [OUTER] | FULL [OUTER] JOIN
     DatabaseName!]Table [[AS] Local_Alias]
     [ON JoinCondition …]

[[INTO Destination]
  | [TO FILE FileName [ADDITIVE] | TO PRINTER [PROMPT]
  | TO SCREEN]]

[PREFERENCE PreferenceName]

[NOCONSOLE]

[PLAIN]

[NOWAIT]

[WHERE JoinCondition [AND JoinCondition ...]
  [AND | OR FilterCondition [AND | OR FilterCondition ...]]]

[GROUP BY GroupColumn [, GroupColumn ...]]

[HAVING FilterCondition]

[UNION [ALL] SELECTCommand]

[ORDER BY Order_Item [ASC | DESC] [, Order_Item [ASC | DESC] ...]]

Arguments

SELECT

Specifies the fields, constants, and expressions that are displayed in the query results.

ALL

By default, displays all the rows in the query results.

DISTINCT

Excludes duplicates of any rows from the query results.

Note   You can use DISTINCT only once per SELECT clause.

TOP nExpr [PERCENT]

Specifies that the query result contains a specific number of rows or a percentage of rows in the query result. You must include an ORDER BY clause when you include the TOP clause. The ORDER BY clause specifies the columns on which the TOP clause determines the number of rows to include in the query result.

You can specify from 1 to 32,767 rows. Rows with identical values for the columns specified in the ORDER BY clause are included in the query result. Therefore, if you specify 10 for nExpr, the query result can contain more than 10 rows if there are more than 10 rows with identical values for the columns specified in the ORDER BY clause.

If the PERCENT keyword is included, the number of rows returned in the result is rounded up to the next highest integer. Permissible values for nExpr when the PERCENT keyword is included are 0.01 to 99.99.

Alias.

Qualifies matching item names. Each item you specify with Select_Item generates one column of the query results. If two or more items have the same name, include the table alias and a period before the item name to prevent columns from being duplicated.

Select_Item specifies an item to be included in the query results. An item can be one of the following:

AS Column_Name

Specifies the heading for a column in the query output. This option is useful when Select_Item is an expression or contains a field function and you want to give the column a meaningful name. Column_Name can be an expression but cannot contain characters (for example, spaces) that aren't permitted in table field names.

FROM

Lists the tables containing the data that the query retrieves. If no table is open, Visual Practice displays the Open dialog box so you can specify the file location. Once open, the table remains open once the query is complete.

FORCE specifies that tables are joined in the order in which they appear in the FROM clause. If FORCE is omitted, Visual Practice attempts to optimize the query. However, the query might be executed faster by including the FORCE keyword to disable the Visual Practice query optimization.

DatabaseName!

Specifies the name of a non-current database containing the table. You must include the name of database containing the table if the database is not the current database. Include the exclamation point (!) delimiter after the database name and before the table name.

[AS] Local_Alias

Specifies a temporary name for the table named in Table. If you specify a local alias, you must use the local alias in place of the table name throughout the SELECT statement.

INNER JOIN specifies that the query result contains only rows from a table that match one or more rows in another table.

LEFT [OUTER] JOIN specifies that the query result contains all rows from the table to the left of the JOIN keyword and only matching rows from the table to the right of the JOIN keyword. The OUTER keyword is optional; it can be included to emphasize that an outer join is created.

RIGHT [OUTER] JOIN specifies that the query result contains all rows from the table to the right of the JOIN keyword and only matching rows from the table to the left of the JOIN keyword. The OUTER keyword is optional; it can be included to emphasize that an outer join is created.

FULL [OUTER] JOIN specifies that the query result contains all matching and non matching rows from both tables. The OUTER keyword is optional; it can be included to emphasize that an outer join is created.

ON JoinCondition specifies the conditions for which the tables are joined.

INTO Destination

Specifies where to store the query results. If you include the INTO clause and the TO clause in the same query, the TO clause is ignored. If you don't include the INTO clause, query results are displayed in a Browse window. You can also use TO to direct query results to the printer or a file.

Destination can be one of the following clauses:

Include NOFILTER to create a cursor that can be used in subsequent queries. In previous versions of Visual FoxPro, it was necessary to include an extra constant or expression as a filter to create a cursor that could be used in subsequent queries. For example, adding a logical true as a filter expression created a query that could be used in subsequent queries: 

SELECT *, .T. FROM customers INTO CURSOR myquery

Including NOFILTER can reduce query performance because a temporary table is created on disk. The temporary table is deleted from disk when the cursor is closed.

Include DATABASE DatabaseName to specify a database to which the table is added. Include NAME LongTableName to specify a long name for the table. Long names can contain up to 128 characters and can be used in place of short file names in the database.

TO FILE FileName

If you include a TO clause but not an INTO clause, you can direct query results to an ASCII text file named FileName, to the printer, or to the main Visual Practice window.

ADDITIVE appends query output to the existing contents of the text file specified in TO FILE FileName.

TO PRINTER [PROMPT] directs query output to a printer. Use the optional PROMPT clause to display a dialog box before printing starts. In this dialog box, you can adjust printer settings. The printer settings that you can adjust depend on the currently installed printer driver. Place PROMPT immediately after TO PRINTER.

TO SCREEN directs query output to the main Visual Practice window or to an active user-defined window.

PREFERENCE PreferenceName

Saves the Browse window's attributes and options for later use, if query results are sent to a Browse window. PREFERENCE saves the attributes, or preferences, indefinitely in the FOXUSER resource file. Preferences can be retrieved at any time.

Issuing SELECT with a PREFERENCE PreferenceName for the first time creates the preference. Issuing SELECT later with the same preference name restores the Browse window to that preference state. When the Browse window is closed, the preference is updated.

If you exit a Browse window by pressing CTRL+Q+W, changes you've made to the Browse window are not saved to the resource file.

NOCONSOLE

Prevents display of query results sent to a file, the printer, or the main Visual Practice window.

PLAIN

Prevents column headings from appearing in the query output that is displayed. You can use PLAIN whether or not a TO clause is present. If an INTO clause is included, PLAIN is ignored.

NOWAIT

Continues program execution after the Browse window is opened and query results are directed to it. The program doesn't wait for the Browse window to be closed, but continues execution on the program line immediately following the SELECT statement.

When TO SCREEN is included to direct output to the main Visual Practice window or to a user-defined window, output pauses when the main Visual Practice window or user-defined window is full of query results. Press a key to see the next set of query results. If NOWAIT is included, the query results are scrolled off the main Visual Practice window or the user-defined window without pausing for a key press. NOWAIT is ignored if included with the INTO clause.

WHERE

Tells Visual Practice to include only certain records in the query results. WHERE is required to retrieve data from multiple tables.

JoinCondition

specifies fields that link the tables in the FROM clause. If you include more than one table in a query, you should specify a join condition for every table after the first.

You must use the AND operator to connect multiple join conditions. Each join condition has the following form:

FieldName1 Comparison FieldName2

FieldName1 is the name of a field from one table, FieldName2 is the name of a field from another table, and Comparison is one of the following operators:

Operator

Comparison

=

Equal

==

Exactly equal

LIKE

SQL LIKE

<>, !=, #

Not equal

>

More than

>=

More than or equal to

<

Less than

<=

Less than or equal to

 

When you use the = operator with strings, it acts differently depending on the setting of SET ANSI. When SET ANSI is set to OFF, Visual Practice treats string comparisons in a manner familiar to Xbase users. When SET ANSI is set to ON, Visual Practice follows ANSI standards for string comparisons. See SET ANSI and SET EXACT for additional information about how Visual Practice performs string comparisons.

The WHERE clause supports the ESCAPE operator for the JoinCondition, allowing you to perform meaningful queries on data containing the SELECT - SQL % and _ wildcard characters.

The ESCAPE clause lets you specify that a SELECT - SQL wildcard character be treated as a literal character. In the ESCAPE clause you specify a character, which when placed immediately before the wildcard character, indicates that the wildcard character be treated as a literal character.

FilterCondition

specifies the criteria that records must meet to be included in the query results. You can include as many filter conditions as you like in a query, connecting them with the AND or OR operator. You can also use the NOT operator to reverse the value of a logical expression, or use EMPTY( ) to check for an empty field.

FilterCondition can take any of the forms in the following examples:

Example 1

Example 1 displays FilterCondition in the form of FieldName1 Comparison FieldName2 

customer.cust_id = orders.cust_id

Example 2

Example 2 displays FilterCondition in the form of FieldName Comparison Expression 

payments.amount >= 1000

Example 3

Example 3 displays FilterCondition in the form of FieldName Comparison ALL (Subquery)

When the filter condition includes ALL, the field must meet the comparison condition for all values generated by the subquery before its record is included in the query results. 

company < ALL ;




(SELECT company FROM customer WHERE country = "UK")

Example 4

Example 4 displays FilterCondition in the form of FieldName Comparison ANY | SOME (Subquery)

When the filter condition includes ANY or SOME, the field must meet the comparison condition for at least one of the values generated by the subquery. 

company < ANY ;




(SELECT company FROM customer WHERE country = "UK")

Example 5

Example 5 displays FilterCondition in the form of FieldName [NOT] BETWEEN Start_Range AND End_Range

This example checks to see whether the values in the field are within a specified range of values. 

customer.postalcode BETWEEN 90000 AND 99999

Example 6

Example 6 displays FilterCondition in the form of [NOT] EXISTS (Subquery)

This example checks to see whether at least one row meets the criterion in the subquery. When the filter condition includes EXISTS, the filter condition evaluates to true (.T.) unless the subquery evaluates to the empty set. 

EXISTS ; 




(SELECT * FROM orders WHERE customer.postalcode =    orders.postalcode)

Example 7

Example 7 displays FilterCondition in the form of FieldName [NOT] IN Value_Set

When the filter condition includes IN, the field must contain one of the values before its record is included in the query results. 

customer.postalcode NOT IN ("98052","98072","98034")

Example 8

Example 8 displays FilterCondition in the form of FieldName [NOT] IN (Subquery)

Here, the field must contain one of the values returned by the subquery before its record is included in the query results. 

customer.cust_id IN ; 




(SELECT orders.cust_id FROM orders WHERE orders.city="Seattle")

Example 9

Example 9 displays FilterCondition in the form of FieldName [NOT] LIKE cExpression 

customer.country NOT LIKE "UK"

This filter condition searches for each field that matches cExpression.

You can use the percent sign (%) and underscore ( _ ) wildcards as part of cExpression. The percent sign represents any sequence of unknown characters in the string. An underscore represents a single unknown character in the string.

GROUP BY GroupColumn [, GroupColumn ...]

Groups rows in the query based on values in one or more columns. GroupColumn can be the name of a regular table field, a field that includes a SQL field function, or a numeric expression indicating the location of the column in the result table (the leftmost column number is 1).

HAVING FilterCondition

Specifies a filter condition which groups must meet to be included in the query results. HAVING should be used with GROUP BY. It can include as many filter conditions as you like, connected with the AND or OR operators. You can also use NOT to reverse the value of a logical expression.

FilterCondition cannot contain a subquery.

A HAVING clause without a GROUP BY clause acts like a WHERE clause. You can use local aliases and field functions in the HAVING clause. Use a WHERE clause for faster performance if your HAVING clause contains no field functions. Note that the HAVING clause should appear before an INTO clause or a syntax error occurs.

[UNION [ALL] SELECTCommand]

Combines the final results of one SELECT with the final results of another SELECT. By default, UNION checks the combined results and eliminates duplicate rows. Use parentheses to combine multiple UNION clauses.

ALL prevents UNION from eliminating duplicate rows from the combined results.

UNION clauses follow these rules:

ORDER BY Order_Item

Sorts the query results based on the data in one or more columns. Each Order_Item must correspond to a column in the query results and can be one of the following:

ASC specifies an ascending order for query results, according to the order item or items, and is the default for ORDER BY.

DESC specifies a descending order for query results.

Query results appear unordered if you don't specify an order with ORDER BY.

Remarks

SELECT is a SQL command that is built into Visual Practice like any other Visual Practice command. When you use SELECT to pose a query, Visual Practice interprets the query and retrieves the specified data from the tables. You can create a SELECT query from within:

When you issue SET TALK ON and execute SELECT, Visual Practice displays the length of time the query took to execute and the number of records in the results. _TALLY contains the number of records in the query results.

SELECT does not respect the current filter condition specified with SET FILTER.

A subquery, referred to in the following arguments, is a SELECT within a SELECT and must be enclosed in parentheses. You can have up to two subqueries at the same level (not nested) in the WHERE clause (see that section of the arguments). Subqueries can contain multiple join conditions.

When you create query output, columns are named according to the following rules:

User-Defined Functions with SELECT   Although using user-defined functions in the SELECT clause has obvious benefits, you should also consider the following restrictions:

Apart from these restrictions, user-defined functions are acceptable in the SELECT clause. However, don't forget that using SELECT might slow performance.

The following field functions are available for use with a select item that is a field or an expression involving a field:

You cannot nest field functions.

Joins   Visual Practice supports ANSI SQL '92 Join syntax, allowing you to create queries that link the rows in two or more tables by comparing the values in specified fields. For example, an inner join selects rows from two tables only when the values of the joined fields are equal. Visual Practice supports nested joins.

Because SQL is based on mathematical set theory, each table can be represented as a circle. The ON clause that specifies the join conditions determines the point of overlap which represents the set of rows that match. For an inner join, the overlap occurs within the interior or "inner" portion of the two circles. An outer join includes not only those matched rows found in the inner cross section of the tables, but also the rows in the outer part of the circle to the left, or right, of the intersection.

Important   Keep the following information in mind when creating join conditions: