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The Query
A query is a SELECT command that describes to ALLBASE/SQL the data you
want retrieved. You can retrieve all or only certain data from a table.
You can have ALLBASE/SQL group or order the rows you retrieve or perform
certain calculations or comparisons before presenting data to your
program. You can retrieve data from multiple tables. You can also
retrieve data using views or combinations of tables and views.
The SELECT Command
The SELECT command identifies the columns and rows you want in your query
result as well as the tables and views to use for data access. The
columns are identified in the select list. The rows are identified in
several clauses (GROUP BY, HAVING, and ORDER BY). The tables and views to
access are identified in the FROM clause. Data thus specified is
returned into host variables named in the INTO clause, as the following
syntax shows:
EXEC SQL SELECT SelectList
INTO HostVariables
FROM TableNames
WHERE SearchCondition1
GROUP BY ColumnName
HAVING SearchCondition2
ORDER BY ColumnID;
To retrieve all data from a table, the SELECT command need specify only
the following:
EXEC SQL BULK SELECT *
INTO :MyArray
FROM PurchDB.Parts;
Although the shorthand notation * can be used in the select list to
indicate you want all columns from one or more tables or views, it is
better programming practice to explicitly name columns. Then, if the
tables or views referenced are altered, your program will still retrieve
only the data its host variables are designed to accommodate:
EXEC SQL BULK SELECT PartNumber,
PartName,
SalesPrice
INTO :MyArray
FROM PurchDB.Parts;
The SELECT command has several clauses you can use to format the data
retrieved from any table:
* The WHERE clause specifies a search condition. A search condition
consists of one or more predicates. A predicate is a test each
row must pass before it is returned to your program.
* The GROUP BY clause and the HAVING clause tell how to group rows
retrieved before applying any aggregate function in the select
list to each group of rows.
* The ORDER BY clause causes ALLBASE/SQL to return rows in ascending
or descending order, based on the value in one or more columns.
The following SELECT command contains a WHERE clause that limits rows
returned to those not containing a salesprice; the predicate used in the
WHERE clause is known as the null predicate:
EXEC SQL BULK SELECT PartName,
SalesPrice
INTO :MyArray
FROM PurchDB.Parts
WHERE SalesPrice IS NULL;
In the UPDATE and DELETE commands, you may need a WHERE clause to limit
the rows ALLBASE/SQL changes or deletes. In the following case, the
sales price of parts priced lower than $1000 is increased 10 percent; the
WHERE clause in this case illustrates the comparison predicate:
EXEC SQL UPDATE PurchDB.Parts
SET SalesPrice = SalesPrice * 1.1
WHERE SalesPrice < 1000.00;
The ALLBASE/SQL Reference Manual details the syntax and semantics for
these and other predicates.
When you use an aggregate function in the select list, you can use the
GROUP BY clause to indicate how ALLBASE/SQL should group rows before
applying the function. You can also use the HAVING clause to limit the
groups to only those satisfying certain criteria. The following SELECT
command will produce a query result containing two columns: a sales
price and a number indicating how many parts have that price:
EXEC SQL BULK SELECT SalesPrice,
COUNT(PartNumber)
INTO :MyArray
FROM PurchDB.Parts
GROUP BY SalesPrice
HAVING AVG(SalesPrice) > 1500.00;
The GROUP BY clause in this example causes ALLBASE/SQL to group all parts
with the same sales price together. The HAVING clause causes ALLBASE/SQL
to ignore any group having an average sales price less than or equal to
$1500.00. Once the groups have been defined, ALLBASE/SQL applies the
aggregate function COUNT to each group.
Null values in a GROUP BY column constitute their own group. Therefore,
a query result having a null value in the column used to group rows would
contain a separate row for each null value.
An aggregate function is one example of an ALLBASE/SQL expression. An
expression specifies a value. An expression can be used in several
places in the SELECT command as well as in the other data manipulation
commands. Refer to the ALLBASE/SQL Reference Manual for the syntax and
semantics of expressions, as well as the effect of null values on them.
The rows in the query result obtained with the preceding query could be
returned in a specific order by using the ORDER BY clause. In the
following case, the rows are returned in descending sales price order:
EXEC SQL BULK SELECT SalesPrice,
COUNT(PartNumber)
INTO :MyArray
FROM PurchDB.Parts
GROUP BY SalesPrice
HAVING AVG(SalesPrice) > 1500.00
ORDER BY SalesPrice DESC;
The examples shown so far have all included the BULK option and a host
variable array, because the query results would most likely contain more
than one row. Besides the BULK table processing technique, the
sequential table processing technique could also be used to handle
multiple-row query results. Later in this chapter you'll find examples
of both these techniques, as well as examples illustrating simple data
manipulation, in which only one row query results are expected.
Selecting from Multiple Tables
To retrieve data from more than one table or view, the query describes to
ALLBASE/SQL how to join the tables before deriving the query result in
the following places:
* In the FROM clause, you identify the tables and views to be
joined.
* In the WHERE clause, you specify a join condition. A join
condition defines the condition(s) under which rows should be
joined.
To obtain a query result consisting of the name of each part and its
quantity-on-hand, you need data from two tables in the sample database:
PurchDB.Parts and PurchDB.Inventory. The join condition in this case is
that you want ALLBASE/SQL to join rows in these tables that have the same
part number:
EXEC SQL BULK SELECT PartName,
QtyOnHand
INTO :MyArray
FROM PurchDB.Parts,
PurchDB.Inventory
WHERE PurchDB.Parts.PartNumber =
PurchDB.Inventory.PartNumber;
Whenever two or more columns in a query have the same name but belong to
different tables, you avoid ambiguity by qualifying the column names with
table names. Because the columns specified in the join condition shown
above have the same name (PartNumber) in both tables, they are fully
qualified with table names (PurchDB.Parts and PurchDB.Inventory). If one
of the columns named PartNumber were named PartNum, the WHERE clause
could be written as follows:
WHERE PartNumber = PartNum
ALLBASE/SQL creates a row for the query result whenever the PartNumber
value in one table matches that in the second table. As illustrated in
Figure 6-1 , any row containing a null PartNumber is excluded from the
join, as are rows that have a PartNumber value in one table, but not the
other.
![[]](../../pic3k/M012400/FFN643c50.gif)
Figure 6-1. Sample Query Joining Multiple Tables
You can also join a table to itself. This type of join is useful when
you want to identify values within one table that have certain
relationships.
The PurchDB.SupplyPrice table contains the unit price, delivery time, and
other data for every vendor that supplies any part. Most parts are
supplied by more than one vendor, and prices vary with vendor. You can
join the PurchDB.SupplyPrice table to itself in order to identify for
which parts the difference among vendor prices is greater than $50. The
query and its result would appear as follows:
EXEC SQL BULK SELECT X.PartNumber,
X.VendorNumber,
X.UnitPrice,
Y.VendorNumber,
Y.UnitPrice
INTO :MyArray
FROM PurchDB.SupplyPrice X,
PurchDB.SupplyPrice Y
WHERE X.PartNumber = Y.PartNumber AND
X.UnitPrice > (Y.UnitPrice + 50.00);
----------------+------------+--------------+------------+--------------
PARTNUMBER |VENDORNUMBER|UNITPRICE |VENDORNUMBER|UNITPRICE
----------------+------------+--------------+------------+--------------
1123-P-01 | 9007| 550.00| 9002| 450.00
1123-P-01 | 9012| 525.00| 9002| 450.00
1123-P-01 | 9007| 550.00| 9008| 475.00
1123-P-01 | 9007| 550.00| 9003| 475.00
1433-M-01 | 9007| 700.00| 9003| 645.00
1623-TD-01 | 9011| 1800.00| 9015| 1650.00
|_________________________|
|
These vendors charge at least $50
more for a part than the vendors
identified in the next two columns.
To obtain such a query result, ALLBASE/SQL joins one copy of the table
with another copy of the table, using the join condition specified in the
WHERE clause:
* You name each copy of the table in the FROM clause by using a
correlation name. In this example, the correlations names are X
and Y. Then you use the correlation name to qualify column names
in the select list and other clauses in the query.
* The join condition in this example specifies that for each part
number, the query result should contain a row only when the price
of the part from vendor to vendor differs by more than $50.
Join variables can be used in any query as a shorthand way of referring
to a table, but they must be used in queries that join a table to itself
so that ALLBASE/SQL can distinguish between the two copies of the table.
Selecting Using Views
Views are used to restrict data visibility as well as to simplify data
access:
* Data visibility can be limited using views by defining them such
that only certain columns and/or rows are accessible through them.
* Data access can be simplified using views by creating views based
on joins or containing columns that are derived from expressions
or aggregate functions.
The sample database has a view called PurchDB.VendorStatistics, defined
as follows:
CREATE VIEW PurchDB.VendorStatistics
(VendorNumber,
VendorName,
OrderDate,
OrderQuantity,
TotalPrice)
AS
SELECT PurchDB.Vendors.VendorNumber,
PurchDB.Vendors.VendorName,
OrderDate,
OrderQty,
OrderQty * PurchasePrice
FROM PurchDB.Vendors,
PurchDB.Orders,
PurchDB.OrderItems
WHERE PurchDB.Vendors.VendorNumber =
PurchDB.Orders.VendorNumber
AND PurchDB.OrderItems.OrderNumber =
PurchDB.OrderItems.OrderNumber
This view combines information from three base tables to provide a
summary of data on existing orders with each vendor. One of the columns
in the view consists of a computed expression: the total cost of an item
on order with the vendor.
Note that the select list of the SELECT command defining this view
contains some qualified and some unqualified column names. Columns
OrderDate, OrderQty, and PurchasePrice need not be qualified, because
these names are unique among the column names in the three tables joined
in this view. In the WHERE clause, however, both join conditions must
contain fully qualified column names, since the columns are named the
same in each of the joined tables.
You can use a view in a query without restriction. In the FROM clause,
you identify the view as you would identify a table. When you reference
columns belonging to the view, you use the column names used in the view
definition. In the view above, for example, the column containing
quantity-on-order is called OrderQuantity, not OrderQty as it is in the
base table (PurchDB.OrderItems).
The VendorStatistics view can be used to quickly determine the total
dollar amount of orders existing for each vendor. Because the view
definition contains all the details for deriving this information, the
query based on this view is quite simple:
EXEC SQL BULK SELECT VendorNumber,
SUM(TotalPrice)
INTO :MyArray
FROM PurchDB.VendorStatistics
GROUP BY VendorNumber;
The query result appears as follows:
------------+----------------------
VENDORNUMBER|(EXPR)
------------+----------------------
9001| 31300.00
9002| 6555.00
9003| 6325.00
9004| 2850.00
9006| 2010.00
9008| 12460.00
9009| 7750.00
9010| 9180.00
9012| 12280.00
9013| 8270.00
9014| 2000.00
9015| 17550.00
Although you can use views in queries without restriction, you can use
only some views to insert, update, or delete rows:
* You cannot INSERT, UPDATE, or DELETE using a view if the view
definition contains one of the following:
* Join operation.
* Aggregate function.
* DISTINCT option.
* GROUP BY clause.
* ORDER BY clause.
* UNION.
* You cannot INSERT using a view if any column of the view is
computed in an arithmetic expression.
The PurchDB.VendorStatistics view cannot be used for any INSERT, UPDATE,
or DELETE operation because it is based on a three table join and
contains a column (TotalPrice) derived from a multiplication operation.