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:


    EXEC SQL    SELECT SelectList
   1               INTO HostVariables
   2               FROM TableNames
   3              WHERE SearchCondition1
   4           GROUP BY ColumnName
   5             HAVING SearchCondition2
   6           ORDER BY ColumnID

To retrieve all data from a table, the SELECT command need specify only the following:


    EXEC SQL SELECT  *
   1            INTO :HostVariable1
   2                 :HostVariable2
   3                  .
   4                  .
   5                  .
   6            FROM  OwnerName.TableName

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 SELECT  PartNumber,
   1                  PartName,
   2                  SalesPrice
   3            INTO :PartNumber,
   4                 :PartName,
   5                 :SalesPrice
   6            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 ALLBASE/SQL 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 SELECT  PartName,
   1                  SalesPrice
   2            INTO :PartName,
   3                 :SalesPrice
   4            FROM  PurchDB.Parts
   5           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
   1             SET SalesPrice = SalesPrice * 1.1
   2           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 SELECT  SalesPrice,
   1                  COUNT(PartNumber)
   2           INTO  :SalesPrice,
   3                 :Count
   4           FROM   PurchDB.Parts
   5       GROUP BY   SalesPrice
   6         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.

Each null value in a GROUP BY column constitutes a separate group. Therefore a query result having a null value in the column(s) 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 SELECT   SalesPrice,
   1                   COUNT(PartNumber)
   2             INTO :SalesPrice,
   3                  :Count
   4             FROM  PurchDB.Parts
   5         GROUP BY  SalesPrice
   6           HAVING  AVG(SalesPrice) > 1500.00
   7         ORDER BY  SalesPrice DESC

The examples shown so far have all included queries where results would most likely contain more than one row. The sequential table processing technique using cursors could also be used to handle multiple-row query results. Later in this chapter you'll find examples of this technique, 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 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 SELECT   PartName,
   1                   QtyOnHand
   2             INTO :PartName,
   3                  :QtyOnHand
   4             FROM  PurchDB.Parts,
   5                   PurchDB.Inventory
   6            WHERE  PurchDB.Parts.PartNumber =
   7                   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 and owner 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 and owner names (PurchDB.Parts and PurchDB.Inventory). If one of the columns named PartNumber were named PartNum, the WHERE clause could be written without having the fully qualified column name 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. 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:

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 pairs of 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:

The query:


    EXEC SQL SELECT  X.PartNumber,
   1                 X.VendorNumber,
   2                 X.UnitPrice,
   3                 Y.VendorNumber,
   4                 Y.UnitPrice
   5           INTO :PartNumber,
   6                :VendorNumber1,
   7                :UnitPrice1,
   8                :VendorNumber2,
   9                :UnitPrice2
   1           FROM  PurchDB.SupplyPrice X,
   2                 PurchDB.SupplyPrice Y
   3          WHERE  X.PartNumber = Y.PartNumber AND
   4                 X.UnitPrice  > (Y.UnitPrice + 50.00)

The result:


   ----------------+------------+--------------+------------+--------------
   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 join variable. In this example, the join variables are X and Y. Then you use the join variable 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:


    EXEC SQL CREATE VIEW  PurchDB.VendorStatistics
   1             (VendorNumber,
   2              VendorName,
   3              OrderDate,
   4              OrderQuantity,
   5              TotalPrice)
   6          AS
   7      SELECT  PurchDB.Vendors.VendorNumber,
   8              PurchDB.Vendors.VendorName,
   9              OrderDate,
   1              OrderQty,
   2              OrderQty * PurchasePrice
   3        FROM  PurchDB.Vendors,
   4              PurchDB.Orders,
   5              PurchDB.OrderItems
   6       WHERE  PurchDB.Vendors.VendorNumber   =
   7              PurchDB.Orders.VendorNumber    AND
   8              PurchDB.OrderItems.OrderNumber =
   9              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 SELECT  VendorNumber,
   1                  SUM(TotalPrice)
   2            INTO :VendorNumber,
   3                 :Sum
   4            FROM  PurchDB.VendorStatistics
   5        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
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.

Query Efficiency

Three clauses in the SELECT command have an effect on the execution speed of queries:


   WHERE
   GROUP BY
   ORDER BY

As discussed earlier, the WHERE clause consists of one or more predicates. Predicates can be evaluated more quickly when they can be optimized by ALLBASE/SQL.

The following predicates are optimizable when all the data types within them are the same (in the case of DOUBLE PRECISION data, the precisions and scales of the different values must be the same). Note that after optimization, ALLBASE/SQL may perform an index scan to access data; an index scan improves data access speed by making use of an index on one or more of the columns in the predicate:

WHERE Column1 ComparisonOperator Column2 where ComparisonOperator is one of the following: =, >, >=, <, or <=. An index may be used if Column1 and Column2 are in different tables and an index exists on either column:
WHERE PurchDB.Parts.PartNumber = PurchDB.SupplyPrice.PartNumber
WHERE Column1 ComparisonOperator {Constant or HostVariable} where ComparisonOperator is one of the following: =, >, >=, <, or <=. An index may be used if one exists on Column1; however, an index may be used if a host variable appears in the predicate only if the comparison operator is an equal sign (=) :
WHERE SupplyPrice = :SupplyPrice
WHERE Column1 BETWEEN {Column2 or Constant or HostVariable} AND {Column2 or Constant or HostVariable}. An index may be used if Column1 is the only column name in the predicate and an index exists on it.
WHERE Column1 <> {Column2 or Constant or Host Variable} Although this kind of predicate is optimizable, an index is never used:
WHERE VendorState <> :VendorState

The lower the cluster count of an index, the greater the chance ALLBASE/SQL will use it when an appropriate index is available. Cluster count indicates the number of times ALLBASE/SQL has to access a different data page to retrieve the next row during an index scan. Refer to the ALLBASE/SQL Database Administration Guide for information on how to optimize the cluster count of an index.

The following predicates are not optimizable, and an index is never used:

Predicates containing arithmetic expressions:


        WHERE Column1 >   Column2 *   :HostVariable

LIKE predicates:


        WHERE Column1 LIKE :HostVariable

Predicates joined by the logical operator OR:


        WHERE Column1 = Column2
          OR Column1 > Constant

When a query does not contain a WHERE clause, an index is never used, because all rows from tables in the FROM clause containing columns in the select list qualify:


    EXEC SQL SELECT  *
   1           INTO  :HostVariableList
   2                  .
   3                  .
   4                  .
   5           FROM   OwnerName.TableName

When an index is not used, ALLBASE/SQL performs what is known as a serial scan to locate rows. When a serial scan is performed instead of an index scan, the entire table is locked, regardless of the automatic locking mode of the table.

The optimization and locking ALLBASE/SQL performs for the WHERE clause in the SELECT command also applies to the WHERE clause in the UPDATE and DELETE commands.

When a query contains a GROUP BY and/or an ORDER BY clause, ALLBASE/SQL must sort rows. The time required for sorting increases as the number of qualifying rows increases.

Sorting occurs in DBEFiles associated with the SYSTEM DBEFileSet. Therefore enough file space must be available in this DBEFileSet when the query is executed to accommodate the sort operations. Guidelines on space requirements can be found in the ALLBASE/SQL Database Administration Guide .

The Query

Technical documentation

» Table of Contents

» Index

The SELECT Command

Selecting from Multiple Tables

Selecting Using Views

Query Efficiency