Scalar subqueries

8/7/2019 Scalar subqueries

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Fall 2001 Database Systems 1

Scalar subqueries

A scalar subquery is any expression that produces a

single value (single tuple/ single column)

Examples:

SELECT max(amount) FROM bidsSELECT count(*) FROM items

Scalar subqueries can be used as if they were

constants

in expressions in the WHERE clause, and

to output values in the SELECT clause


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Scalar subqueries

Find items with average bid 60 or more. Print the

name of the item and the average bid.

SELECT I.name, avg(B.amount)

FROM Items I, Bids B

WHERE I.iid=B.iid

GROUP BY I.iid, I.name

HAVING avg(B.amount) > 60


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Scalar subqueries

Find items with average bid 60 or more. Print the name

of the item and the average bid.

SELECT I.name,

FROM Items I

WHERE 60 < (SELECT avg(B.amount)

FROM bids BWHERE B.iid = I.iid)

(SELECT avg(B2.amount)

FROM Bids B2

WHERE B2.iid = I.iid) avgAmount

Return a single

value for each tuple

Check the boolean condition

for each tuple


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What cant you do with SQL

or can you?

You are limited by the functions implemented in the

database

Not allowed: SELECT median(salary)

You can add new functions to the database engine andimplement new types in a native programming language like

C,C++ (support varies)

You can have subselects in FROM and WHERE that

return a set of values, but not in SELECT

Allowed: SELECT * FROM (SELECT b.bid FROM bids bWHERE b.amount>20)

Not allowed: SELECT avg(SELECT b.amount FROM .. ) FROM

..


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What cant you do with SQL

or can you?

You cannot ask transitive queries

Given a relation emp(A,B) for immediate managers B or A, you cannotfind all managers of a person A.

SQL3 includes a recursive query definition construct not implementedin Oracle.

WITH RECURSIVE manages(emp, mgr) AS

(SELECT E1.emp, E1.mgr FROM employee E1)

UNION

(SELECT M.emp, E2.mgr FROM manages M, employee E2

WHERE M.mgr = E.id)

SELECT M2.empFROM manages M2

WHERE M2.emp = M2.mgr ;

Recursive queries may produce infinite loops and it is best left to programmerswith solid logic background.


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Embedded SQL

You can embed SQL statements into many programming

languages procedural power (loops, variables, etc.)

The main components of embedded SQL programming:

Regular program blocks SQL code

Methods to connect to the database, invoke the SQL code and

retrieve results

Methods to pass data from program variables to the SQL code

Methods to retrieve data from the result of queries to programvariables


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#include

exec sql include sqlca;

char user_prompt[] = "Please enter username and password: ";

char cid_prompt[] = "Please enter customer ID: ";

int main()

{

exec sql begin declare section; /* declare SQL host variables */

char cust_id[5];

char cust_name[14];

float cust_discnt; /* host var for discnt value */

char user_name[20], user_pwd[20];

exec sql end declare section;

exec sql whenever sqlerror goto report_error; /* error trap condition */

exec sql whenever not found goto notfound; /* not found condition */

exec sql connect :user_name

identified by :user_pwd; /* ORACLE format: connect */

while (prompt(cid_prompt, 1, cust_id, 4) >= 0) {exec sql select cname,

discnt into :cust_name, :cust_discnt /* retrieve cname, discnt */

from customers where cid = :cust_id;

exec sql commit work; /* release read lock on row */

printf("CUSTOMER'S NAME IS %s AND DISCNT IS %5.1f\n",

cust_name, cust_discnt); /* NOTE, (:) not used here */

continue;


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ProC

Each ESQL statement starts with EXEC SQL keyword

and ends with a semicolon ;

A pre-compiler will scan a program file and only read the

statements enclosed within EXEC SQL statements anddisregard everything else.

SQLCA is a specific data structure for storing status

codes of all SQL operations

/* always have this for error handling*/exec sql include sqlca ;


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Program.pc

Program.c


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Variables in ESQL

All variables that will be used in an SQL statement must be declared

using an ESQL declaration and data type

exec sql begin declare section ;

VARCHAR e_name[30], username[30], passwd[30] ;

INTEGER e_ssn, e_dept_id ;exec sql end declare section ;

You can use almost any SQL command in ESQL as long as proper

input to these commands are provided in the form of program

variables.

To execute any command, you must first connect to a database inwhich all your tables reside.

exec sql connect :username identified by :passwd ;


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Executing SQL commands

Suppose we want to find the name of an employee given his/herSSN (input by the user of the program):

exec sql select name, dept_id into :e_name, :e_dept_idfrom employee

where ssn = :e_ssn ;

Read the value of the variable e_ssn and execute the SQLstatement using this value, store the returned values of columnsname and dept_id in the program variables e_name and

e_dept_id.

Compare the above query with the expression below. What is the difference?

exec sql select name, dept_id

from employee where ssn = e_ssn ;

Program variables arepreceded by :


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Dealing with Strings

There is a mismatch between the definition of a string in Oracle and

in C/C++.

In C, the end of a string is identified by the null character \0. Hence,

Sibel would be stored as characters S,i,b,e,l,\0.

In Oracle, the length of a string is stored together with the string andthere is no special end of string character.

If you convert a data string from Oracle to C, you must pad it with \0

manually!

The data type VARCHAR e_name[30] is translated by the pre-

compiler to the following structure:

struct {

unsigned short len

unsigned char arr[30]

} e_name ;


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More on strings

Putting the pieces together:

strcpy(username.arr, Sibel Adali) ;username.len = strlen(Sibel Adali) ;strcpy(passwd.arr, tweety-bird) ;

passwd.len = strlen(tweety-bird) ;exec sql connect :username identified by :passwd ;scanf(%d, &e_ssn) ;

exec sql select name, dept_id into :e_name, :e_dept_idfrom employee where ssn = :e_ssn ;

e_name.arr[e_name.len] = \0 ; /* so can use string in C*/printf(%s, e_name.arr) ;

exec sql commit work ; /* make any changes permanent */exec sql disconnect ; /* disconnect from the database */


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ESQL - Cursors

When a select statement returns a set of tuples, then the

tuples (rows) can only be retrieved one by one.

programming language variables can contain only one

value at a time

this is sometimes called an impedance mismatch Cursor operations

declare a cursor using a regular SQL query (no into).

exec sql declare emps_dept cursor for

select ssn, name from employee

where dept_id = :e_dept_id ;


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How do we know when we

reach the end of a cursor?

Each SQL statement executed in a program produces

a status code that can be retrieved from the SQL

Communication Area (SQLCA) variables.

Check the sqlcode to see if the end of a cursor is

reached (its expected value depends on the system).if (sqlca.sqlcode == -1) { }

Error handling statements

exec sql whenever sqlerror

{continue, stop, goto label, call function} exec sql whenever is a trap condition, it hold for all

exec sql statements unless a new trap condition

overrides the current one.


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Dynamic SQL

In Dynamic SQL, embedded SQL statements are created on the fly

using strings!

these strings are fed to an exec sql statement

exec sql execute immediate :sql_string

Since dynamic SQL statements are not known to the pre-compiler atcompile time, they must be optimized at run time!

Create a query once using a prepare statement and run it multiple

times using the execute statement.

strcopy(sqltext.arr, delete from employee where ssn = ?) ;

sqltext.len=str.len(sqltext.arr) ;exec sql prepare del_emp from :sqltext ;

exec sql execute del_emp using :cust_id ;


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SQLDA

When we execute a dynamic SQL statement, we do not know which

columns will be returned and how many columns will be returned.

The SQLDA descriptor definition allows us to find the number of

columns and the value for each column.

exec sql include sqlda ;

exec sql declare sel_curs cursor for sel_emps ;

exec sql prepare sel_emps from :sqltext ;

exec sql describe sel_emps into sqlda ;

exec sql open sel_curs ;

exec sql fetch sel_curs using descriptor sqlda ;


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JDBC Dynamic SQL

Driver is a piece of software that enables

communication between a program and a

database system (DMBS specific packages).

It implements a number of main classes: Connection (opening, closing, committing)

Statement (executing queries)

Result set (going through cursors, extracting

information)


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import java.sql.*;

import oracle.sql.*;

import oracle.jdbc.driver.*;

class Employee{

public static void main (String args []) throws SQLException

{//Set your user name and the password

String userName = "dummy" ;

String passWord = "dummy" ;

// Load the Oracle JDBC driver

DriverManager.registerDriver(neworacle.jdbc.driver.OracleDriver());

Connection conn =

DriverManager.getConnection

("jdbc:oracle:thin:@vcmr-57.server.rpi.edu:1521:ora8",userName,passWord);


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// Create a statement which will return a cursor that will allowyou to

// scroll the result set using both "next" and "previous"methods

Statement stmt = conn.createStatement

(ResultSet.TYPE_SCROLL_INSENSITIVE,

ResultSet.CONCUR_READ_ONLY);

ResultSet rset = stmt.executeQuery

("SELECT name, oid FROM items ");

// Iterate through the result and print the item nameswhile (rset.next ()) {

//Get item name, which is the first column

System.out.println (rset.getString (1));

PreparedStatement pstmt = conn.prepareStatement

("SELECT name FROM owners WHERE oid = ?") ;

//Feed the owner id retrieved from rset into pstmt

pstmt.setInt(1, rset.getInt (2));

ResultSet dset = pstmt.executeQuery() ;

if (dset.next())

System.out.println(dset.getString (1));

} } }

Scalar subqueries

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