- Command window
- Visual FoxPro program as with any other Visual FoxPro command
- Query Designer
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] ...]]
Parameters
- SELECT
- Specifies the fields, constants, and expressions displayed in the query results.
- ALL
- Displays all the rows in the query results by default.
- DISTINCT
- Excludes duplicates of any rows from the query results. You can use DISTINCT only once per SELECT clause.
Visual FoxPro does not support using the
DISTINCT clause in a SQL
SELECT statement that contains memo or general fields. Instead, you can wrap a
Memo field expression inside a function such as
PADR( ) or
ALLTRIM( ). For more information, see
PADL( ) | PADR( ) | PADC( ) Functions and
ALLTRIM( ) Function.
- TOP nExpr [PERCENT]
- Specifies the query result contain a specific number of rows or a percentage of rows of the query result. You can specify from 1 to 32,767 rows. If you include the PERCENT option, permissible values for nExpr are 0.01 to 99.99. When you include the PERCENT option, the number of rows returned in the result is rounded up to the next highest integer.
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.
When you include the TOP clause, you must include an ORDER BY clause. The ORDER BY clause specifies the columns on which the TOP clause determines the number of rows to include in the query result.
- [Alias.] Select_Item
- Qualifies matching item names. Select_Item specifies an item to be included in the query results. An item can be one of the following:
- The name of a field from a table in the FROM clause.
- A constant specifying that the same constant value appears in every row of the query results.
- An expression that can be the name of a user-defined function.
For more information about using user-defined functions, see User-Defined Functions with SELECT in the Remarks section.
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.
- [AS] Column_Name
- Specifies the heading for a column in the query output. Column_Name can be an expression but cannot contain characters that are not permitted, for example, spaces, in table field names.
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.
- FROM [FORCE] DatabaseName!
- Lists the tables containing the data that the query retrieves.
FORCE specifies that tables are joined in the order in which they appear in the FROM clause. If FORCE is omitted, Visual FoxPro attempts to optimize the query. However, the query might be executed faster by including the FORCE keyword to disable the Visual FoxPro 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 contain 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 JOINkeyword and only matching rows from the table to the right of the JOIN keyword. The OUTER keyword is optional; you can include it to emphasize that an outer join is created.
RIGHT [OUTER] JOIN specifies that the query result contain all rows from the table to the right of the JOINkeyword 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 contain all matching and nonmatching rows from both tables. The OUTER keyword is optional; you can include it to emphasize that an outer join is created.
For more information about joins, see Joins in the Remarks section.
ON JoinCondition specifies the conditions for which the tables are joined.
- INTO Destination
- Specifies where to store the query results. Destination can be one of the following clauses:
- ARRAY ArrayName stores query results in a memory variable array.
The array is not created if the query selects 0 records.
- CURSOR CursorName [NOFILTER | READWRITE] stores query results in a cursor.
To create a cursor that can be used in subsequent queries, use NOFILTER. For more information aboutNOFILTER, see the Remarks section.
To specify that the cursor is temporary and modifiable, use READWRITE. If the source table or tables use autoincrementing, the settings are not inherited by the READWRITE cursor.
- DBF | TABLE TableName [DATABASE DatabaseName [NAME LongTableName]] stores query results in a table.
To specify a database to which the table is added, include DATABASE DatabaseName.
To specify a long name for the table, include NAME LongTableName. Long names can contain up to 128 characters and can be used in place of short file names in the database.
If you do not include the INTO clause, query results are displayed in a Browse window. You can also use theTO FILE clause to direct the query results to the printer or a file.
- TO FILE FileName [ADDITIVE] | TO PRINTER [PROMPT] | TO SCREEN
- Directs the query results to the printer or a file.
ADDITIVE appends query output to the existing contents of the text file specified in TO FILE FileName.
TO PRINTER directs query output to a printer. To display a dialog box before printing starts, use the PROMPTclause immediately after TO PRINTER. You can adjust printer settings in the dialog box. The printer settings that you can adjust depend on the currently installed printer driver.
TO SCREEN directs query output to the main Visual FoxPro window or to an active user-defined window.
- PREFERENCE PreferenceName
- Saves the attributes and options of the Browse window for later use, if query results are sent to a Browse window. For more information about how PREFERENCE functions, see the Remarks section.
- NOCONSOLE
- Prevents display of query results sent to a file, the printer, or the main Visual FoxPro 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 does not wait for the Browse window to close but continues execution on the program line immediately following the SELECT statement. For an explanation of how you can use NOWAIT, see the Remarks section.
- WHERE JoinCondition
- Specifies that Visual FoxPro include only records in the query results that meet specified criteria. JoinConditionspecifies fields that link the tables in the FROM clause. For more information about specifying join conditions, see the Remarks section.
WHERE supports the ESCAPE operator for JoinCondition, making it possible for you to perform meaningful queries on data containing the SELECT SQL command percent (%) and underscore (_) wildcard characters.ESCAPE allows you to specify that a SELECT SQL command 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 and connect them with the AND or OR operator. To reverse the value of a logical expression, use the NOT operator. To check for an empty field, use the EMPTY( ) function.
The SELECT SQL command supports "<field> IS / IS NOT NULL" in the filter condition. To learn how to use theFilterCondition, see the Examples section.
- 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 an 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 that groups must meet to be included in the query results. The HAVING clause can include as many filter conditions as you like, connected with the AND or OR operators. To reverse the value of a logical expression, use NOT. You can use local aliases and field functions in the HAVING clause. For more information on the field functions you can use, see the Remarks section. FilterCondition cannot contain a subquery.
You can use a SELECT statement containing a HAVING clause without including a GROUP BY clause if SELECTdoes not contain aggregate functions. The HAVING clause without a GROUP BY clause acts like the WHEREclause. If the HAVING clause contains no field functions, use the WHERE clause for faster performance.
The HAVING clause should appear before an INTO clause or a syntax error occurs.
- [UNION [ALL] SELECTCommand]
- Combines the final result for one SELECT statement with the final result of another SELECT statement. UNIONchecks the combined results and eliminates duplicate rows by default. To combine multiple UNION clauses, use parentheses. You can use the UNION clause to simulate an outer join.
ALL prevents UNION from eliminating duplicate rows from the combined results.
When one of the columns has Memo or General type, unions of differing column types should not be allowed.
In versions earlier than Visual FoxPro 8.0, you needed to perform explicit conversion when performing UNIONoperations between two fields of different types. Visual FoxPro now supports implicit data type conversion for data types that support it. For more details about implicit data type conversion and data type precedence, rules that UNION clauses follow, and other information, see Data Type Conversion and Precedence in the Remarks section.
- ORDER BY Order_Item [ASC | DESC]
- 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:
- A field in a FROM table that is also a Select_Item in the main SELECT clause (not a subquery).
- A numeric expression indicating the location of the column in the result table. The leftmost column is number 1.
ASC specifies an ascending order for query results according to the order item or items and acts as the default for ORDER BY.
DESC specifies a descending order for query results.
Remarks
If no table is open when using the FROM clause, Visual FoxPro displays the Open dialog box so you can specify the file location. Once open, the table remains open once the query is complete.
When using the CURSOR clause in the Destination parameter, if you specify the name of an open table, Visual FoxPro generates an error message. After SELECT is executed, the temporary cursor remains open and is active and read-only unless you specify the READWRITE option. When you close this temporary cursor, it is deleted. Cursors can exist as a temporary file on the drive or volume specified by SORTWORK.
When using the CURSOR clause in the Destination parameter, you can now use NOFILTER to create a cursor that can be used in subsequent queries. In previous versions of Visual FoxPro, you needed to include an extra constant or expression as a filter. 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
However, 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.
When using the DBF | TABLE clause in the Destination parameter, if you specify a table that is already open, and SET SAFETY is set to OFF, Visual FoxPro overwrites the table without warning. If you do not specify an extension, Visual FoxPro gives the table a .dbf extension. The table remains open and active after SELECT is executed.
If you include the INTO and TO clauses in the same query, Visual FoxPro disregards the TO clause. If you include a TOclause 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 FoxPro window.
The PREFERENCE clause saves the attributes, or preferences, indefinitely in the FoxUser.dbf resource file. Preferences can be retrieved at any time. Issuing SELECT with 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 made to the Browse window are not saved to the resource file.
As an example of how you can use the NOWAIT clause, when you include the TO SCREEN clause to direct output to the main Visual FoxPro window or to a user-defined window, output pauses when the main Visual FoxPro window or user-defined window is full of query results. To see the next set of query results, press a key. If you include NOWAIT, the query results scroll off the main Visual FoxPro window or the user-defined window without pausing for a key press. Visual FoxPro disregards NOWAIT if it is included with the INTO clause.
Including the EVALUATE( ) function in the WHERE clause of an SQL query can return incorrect data.
If you include more than one table in a query, you should specify a join condition for every table after the first. Join conditions can include filter conditions.
Note The maximum number of joins per SELECT statement is nine.
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, andComparison 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 FoxPro compares strings only to the end of the shorter string. When
SET ANSI is set to
ON, Visual FoxPro follows ANSI standards for string comparisons. For additional information about how Visual FoxPro performs string comparisons, see
SET ANSI and
SET EXACT.
The following field functions are available for use with a select item that is a field or an expression involving a field:
- AVG(Select_Item), which averages a column of numeric data.
- COUNT(Select_Item), which counts the number of select items in a column. COUNT(*) counts the number of rows in the query output.
- MIN(Select_Item), which determines the smallest value of Select_Item in a column.
- MAX(Select_Item), which determines the largest value of Select_Item in a column.
- SUM(Select_Item), which totals a column of numeric data.
You cannot nest field functions.
UNION clauses follow these rules:
- You cannot use UNION to combine subqueries.
- Both SELECT commands must have the same number of columns in their query output.
- When two columns of different data types are involved in a UNION operation, the data type with the lower precedence is converted to the data type with the higher precedence. For more information, see "Data Type Conversion and Precedence" section in the Remarks section.
- Only the final SELECT statement can have an ORDER BY clause, which must refer to output columns by number. If an ORDER BY clause is included, it affects the entire result.
If you do not specify an order in the ORDER BY clause, query results appear in no order.
When you issue SET TALK ON and execute SELECT, Visual FoxPro 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.
Note A subquery, referred to in the following arguments, is a SELECT within a SELECT and must be enclosed in parentheses. You can have one subquery 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:
- If Select_Item is a field with a unique name, the output column name is the field's name.
- If more than one Select_Item has the same name, an underscore and a letter are appended to the column name. For example, if a table called CUSTOMER has a field called STREET, and a table called EMPLOYEES has a field called STREET also, output columns are named Extension_A and Extension_B (STREET_A and STREET_B). For a select item with a 10-character name, the name is truncated to add the underscore and letter. For example, DEPARTMENT would become DEPARTME_A.
- If Select_Item is an expression, its output column is named EXP_A. Any other expressions are named EXP_B, EXP_C, and so on.
- If Select_Item contains a field function such as COUNT( ), the output column is named CNT_A. If another select item contains SUM( ), its output column is named SUM_B.
User-Defined Functions with SELECT Although using user-defined functions in the SELECT clause has obvious benefits, you should also consider the following restrictions:
- The speed of operations performed with SELECT might be limited by the speed at which such user-defined functions are executed. High-volume manipulations involving user-defined functions may be better accomplished by using API and user-defined functions written in C or assembly language.
- You can assume nothing about the Visual FoxPro input/output (I/O) or table environment in user-defined functions invoked from SELECT. In general, you don't know which work area is selected, the name of the current table, or even the names of the fields being processed. The value of these variables depends on where precisely in the optimization process the user-defined function is invoked.
- It isn't safe to change the Visual FoxPro I/O or table environment in user-defined functions invoked fromSELECT. In general, the results are unpredictable.
- The only reliable way to pass values to user-defined functions invoked from SELECT is by the argument list passed to the function when it is invoked.
- If you experiment and discover a supposedly forbidden manipulation that works correctly in a certain version of FoxPro, there is no guarantee it will continue to work in later versions.
Apart from these restrictions, user-defined functions are acceptable in the SELECT clause. However, do not forget that using SELECT might slow performance. To learn how you can use SELECT with user-defined functions, see the Examples section.
Joins Visual FoxPro 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 FoxPro 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.
Note Keep the following information in mind when creating join conditions:
- If you include two tables in a query and do not specify a join condition, every record in the first table is joined with every record in the second table as long as the filter conditions are met. Such a query can produce lengthy results.
- Be careful when using, in join conditions, functions such as DELETED( ), EOF( ), FOUND( ), RECCOUNT( ), andRECNO( ), which support an optional alias or work area. Including an alias or work area in these functions might yield unexpected results. SELECT doesn't use your work areas; it performs the equivalent ofUSE ... AGAIN. Single-table queries that use these functions without an optional alias or work area will return proper results. However, multiple-table queries that use these functions — even without an optional alias or work area — might return unexpected results.
- Use caution when joining tables that contain empty fields because Visual FoxPro matches empty fields. For example, suppose you perform a join on CUSTOMER.ZIP and INVOICE.ZIP. If CUSTOMER contains 100 empty zip codes, and INVOICE contains 400 empty zip codes, the query output contains 40,000 extra records resulting from the empty fields. To eliminate empty records from the query output, use the EMPTY( ) function.
- The limit to the number of joins that you can use per SELECT statement is nine.
Data Type Conversion and Precedence Explicit data type conversions require you to use Visual FoxPro conversion functions, such as CTOD( ), while implicit conversions do not require you to use conversion functions. Visual FoxPro supports implicit data type conversion for data types that support it. When Visual FoxPro combines two columns of different data types in a SELECT...UNION operation, the data type with the lower precedence is converted to the data type with the higher precedence. For field properties, NULL takes higher precedence over NOT NULL.
