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SDB - a Simple Database System
by David Betz
114 Davenport Ave.
Manchester, NH 03103
(603) 625-4691
Converted to the IBM/PC by
David N. Smith
44 Ole Musket Lane
Danbury, CT 06810
(203) 748-5934
1.0 INTRODUCTION
SDB is a simple database manager for small systems. It was
developed to provide a relatively low overhead system for
storing data on machines with limited disk and memory
resources. The current version runs on a PDT-11/150 with 2
RX01 floppy disk drives and 60K bytes of memory under the
RT-11 operating system. (it also runs on the VAX under VMS)
SDB was originally intended to be a relational database
system, so many of the terms used in describing it are taken
from the relational database literature. Within the context
of SDB the user can safely make the following associations:
1. RELATION can be taken to mean FILE
2. TUPLE can be taken to mean RECORD
3. ATTRIBUTE can be taken to mean FIELD
It should be noted that SDB is not a relationally complete
system. It provides the relational operations of SELECT,
PROJECT, and JOIN, but does not provide the set operations
of UNION, INTERSECTION, or DIFFERENCE as well as some
others.
2.0 RELATION FILE FORMATS
SDB maintains a separate file for each relation that the
user creates. This file contains a header block containing
the definition of the relation including the names and types
of all of the relation's attributes. The remainder of the
file contains fixed length records each containing one tuple
from the relation.
Tuples can be of three types:
1. active - tuples that contain actual active data
2. deleted - tuples that have been deleted
3. unused - tuples that haven't been used yet
SDB - a Simple Database System Page 2
Initially, all tuples are unused. When a new tuple is
stored into a relation, the first unused tuple is found
(they are all contiguous at the end of the relation file).
The new tuple is stored as an active tuple.
When a tuple is deleted, it is marked as such. The space
previously allocated to the deleted tuple is left unused
until the relation is compressed.
It is possible that when attempting to store a new tuple, no
unused tuple can be found even though the relation contains
fewer than the maximum active tuples. This happens when
tuples have been deleted since the time the relation file
was last compressed.
The compress function allows all of the space lost by
deleting tuples to be regained. It does this by copying all
of the active tuples as far backward in the file as possible
leaving all of the available space toward the end of the
file.
3.0 SELECTION EXPRESSIONS
A selection expression specifies a set of tuples over which
some SDB operation is to be executed. The syntax for a
selection expression is:
<rse> ::= <rnames> [ where <boolean> ]
<rnames> ::= <rname> [ , <rname> ] ...
<rname> ::= <relation-name> [ <alias> ]
When a single relation name is specified in a selection
expression, each tuple within that relation becomes a
candidate for selection.
When more than one relation name is specified, the tuples
are formed by taking the cross product of all specified
relations. If a relation is to be crossed with itself, an
alias must be given to one or both of the occurances of that
relation name in the selection expression. This allows SDB
to determine which relation occurance is being refered to in
the boolean part of the selection expression.
After the set of candidate tuples is determined, the boolean
expression is evaluated for each candidate. The candidates
for which the boolean expression evaluates to TRUE become
the selected tuples.
SDB - a Simple Database System Page 3
4.0 INITIALIZATION FILE AND COMMAND FILES
When SDB is first run, it attempts to read and process
commands from a file named "SDB.INI". This file usually
contains macro definitions, but can contain any valid SDB
command. In addition, it is possible to process command
files from within SDB. This is done by typing an '@'
followed by the command file name after the SDB prompt.
5.0 FILE NAMES
Whenever a file name is allowed in the syntax for a command,
it is possible to use either an identifier or a quoted
string. An identifier is interpreted as the file name and a
string is interpreted as a full file specification. The
string form allows for the specification of an alternate
device or extension.
6.0 FORM DEFINITION FILES
A form definition file contains a template into which
attribute values are substituted during a print operation.
There are two types of information that can be included in a
form definition:
1. Literal text
2. Attribute references
Attribute references are indicated by placing the name of
the attribute being referenced between a pair of angle
brackets. Literal text is anything that is not enclosed in
angle brackets.
SDB - a Simple Database System Page 4
Example:
________
print using test amount,category from checks;
Where test.frm contains:
Amount: <amount>
Category: <category>
(MPK) For ease in generating English text, a $ character may
be placed before the attribute name inside an attribute
reference, in which case no trailing blanks will appear in
the output for that reference. For example:
print using test number,category from checks;
Where test.frm contains:
Check #<$number> was spent on <$category
7.0 ALIASES FOR RELATIONS AND ATTRIBUTES
When a relation or attribute name is specified in a print
statement, it is possible to provide an alternate name for
that relation or attribute. This is useful for relations,
when it is necessary to join a relation to itself. It is
useful for attributes when it is desired that the column
headers in a table be different from the actual attribute
names. Also, alternate attribute names can be used in
references to that attribute in the where clause as well as
in a form definition file. The syntax for specifying
aliases is:
<name> <alias>
Example:
________
print using test amount a,category c from checks;
Where test.frm contains:
Amount: <a>
Category: <c>
SDB - a Simple Database System Page 5
8.0 BOOLEAN EXPRESSIONS
The syntax for boolean expressions:
<expr> ::= <land> [ '|' <land> ]
<land> ::= <relat> [ '&' <relat> ]
<relat> ::= <primary> [ <relop> <primary> ]
<primary> ::= <term> [ <addop> <term> ]
<term> ::= <unary> [ <mulop> <unary> ]
<unary> ::= <factor> | <unop> <unary>
<factor> ::= <operand> | '(' <expr> ')'
<operand> ::= <number> | <string> | <attribute>
<attribute> ::= [ <rname> . ] <aname>
<relop> ::= '=' | '<>' | '<' | '>' | '<=' | '>='
<addop> ::= '+' | '-'
<mulop> ::= '*' | '/'
<unop> ::= '+' | '-' | '~'
Operators:
1. '=' - equal
2. '<>' - not equal
3. '<' - less than
4. '>' - greater than
5. '<=' - less than or equal
6. '>=' - greater than or equal
7. '+' - addition or unary plus (not implemented)
8. '-' - subraction or unary minus (not implemented)
9. '*' - multiplication (not implemented)
10. '/' - division (not implemented)
11. '&' - logical and
12. '|' - logical or
13. '~' - logical not
Operands:
1. number - a string of digits containing at most one
decimal point
2. string - a string of characters enclosed in double
quotes
SDB - a Simple Database System Page 6
3. attribute - an attribute name optionally qualified
by a relation name
SDB - a Simple Database System Page 7
9.0 INTERACTIVE COMMAND DESCRIPTIONS
Function:
_________
Create a relation file
Format:
_______
create <rname> ( <alist> ) <size>
Rules:
______
1. <rname> is the name of the relation file
2. <alist> is a list of attribute definitions of the
form:
<aname> { char | num } <size>
where:
1. <aname> is the name of the attribute
2. the type of the attribute is either "char" or
"num"
3. <size> is the number of bytes allocated to the
attribute value
3. <size> is the maximum number of tuples the file is
to hold
Example:
________
create checks (
number num 4
date char 8
payee char 20
amount num 8
category char 5
) 200
This command creates a relation file named "checks.sdb" with
attributes "number", "date", "payee", "amount", and
"category" and space to store 200 tuples.
SDB - a Simple Database System Page 8
Function:
_________
Insert tuples into a relation
Format:
_______
insert <rname>
Rules:
______
1. <rname> is the name of a relation
2. the user will be prompted for the values of the
attributes for the tuple to be inserted
3. a null response to an attribute prompt will
terminate tuple entry
4. if a null value is desired, a single space can be
entered
SDB - a Simple Database System Page 9
Function:
_________
Delete tuples from a set of relations
Format:
_______
delete <rse> ;
Rules:
______
1. <rse> is a tuple selection expression
2. selected tuples are deleted
Example:
________
delete checks where category = "junk";
SDB - a Simple Database System Page 10
Function:
_________
Update the values of selected attributes in selected tuples
Format:
_______
update { <attrs> | * } from <rse> ;
Rules:
______
1. <attrs> is a list of attribute names to be updated
2. * means all attributes
3. <rse> is a tuple selection expression
4. for each set of selected tuples, the user is
prompted for new values for the selected attributes
5. a null response to an attribute prompt will retain
the previous attribute value
6. if a null value is desired, a single space can be
entered
Example:
________
update amount,category from checks where number > 10;
SDB - a Simple Database System Page 11
Function:
_________
Print a table of values of selected attributes
Format:
_______
print [ using <fname> ] { <attrs> | * } from <rse> [ into
<fname> ] ;
Rules:
______
1. using <fname> indicates output using a form
definition file (.FRM)
2. <attrs> is a list of attribute names to be printed
3. * means all attributes
4. <rse> is a tuple selection expression
5. <fname> is the name of an file to which the table
will be output (.TXT)
6. if the output file name is omitted, output is to
the terminal
7. for each set of selected tuples, a table entry is
printed containing the selected attributes
Example:
________
print payee,amount from checks where category = "junk";
SDB - a Simple Database System Page 12
Function:
_________
Import tuples from a file into a relation
Format:
_______
import <fname> into <rname>
Rules:
______
1. <fname> is the name of the input file (.DAT)
2. the input file contains the values of the tuple
attributes with each on a separate line
3. <rname> is the name of a relation
4. tuples are appended to the named relation
SDB - a Simple Database System Page 13
Function:
_________
Export tuples from a relation into a file
Format:
_______
export <rname> [ into <fname> ] ;
Rules:
______
1. <rname> is the name of a relation
2. <fname> is the name of the output file (.DAT)
3. if the output file name is omitted, output is to
the terminal
4. tuples are written to the output file with one
attribute value per line
SDB - a Simple Database System Page 14
Function:
_________
Extract the definition of a relation into a file
Format:
_______
extract <rname> [ into <fname> ] ;
Rules:
______
1. <rname> is the name of a relation
2. <fname> is the name of the output file (.DEF)
3. if the output file name is omitted, output is to
the terminal
4. the definition of the relation is written to the
output file
SDB - a Simple Database System Page 15
Function:
_________
Compress a relation file
Format:
_______
compress <rname>
Rules:
______
1. <rname> is the name of a relation file
2. tuples are copied toward the front of the relation
file such that any space freed by previously
deleted tuples becomes adjacent to the free space
at the end of the file, thus becoming available for
use in inserting new tuples
SDB - a Simple Database System Page 16
Function:
_________
Sort a relation file
Format:
_______
sort <rname> by <sname> { , <sname } ... ;
Rules:
______
1. <rname> is the name of a relation file
2. <sname> is the name of an attribute to sort on
followed optionally by "ascending" or "descending"
3. if a sort order is not specified, ascending is
assumed
4. tuples within the relation are sorted in place
using the attributes indicated
SDB - a Simple Database System Page 17
Function:
_________
Define a macro
Format:
_______
define <mname>
Rules:
______
1. <mname> is the name of the macro being defined
2. if a macro with the specified name already exists,
it is replaced
3. after entering the define command, definition mode
is entered
4. definition mode is indicated by the prompt
"SDB-DEF>"
5. all lines typed in definition mode are added to the
macro definition
6. a blank line terminates definition mode
7. a macro can be deleted by entering a blank line as
the only line in the definition
8. after a macro is defined, every occurance of the
macro name is replaced by the macro definition
SDB - a Simple Database System Page 18
Function:
_________
Show a macro definition
Format:
_______
show <mname>
Rules:
______
1. <mname> is the name of a macro whose definition is
to be shown
SDB - a Simple Database System Page 19
Function:
_________
Print a short help message
Format:
_______
help
Rules:
______
1. (none)
SDB - a Simple Database System Page 20
Function:
_________
Exit from SDB
Format:
_______
exit
Rules:
______
1. (none)
SDB - a Simple Database System Page 21
10.0 PROGRAM INTERFACE
SDB provides a callable program interface to allow programs
written in DECUS-C to access relation files. In order to
use the call interface, the users program should be linked
with the SDBUSR.OBJ object library. Also, additional stack
space should be allocated at link time using the /BOTTOM
qualifier on the link command. /BOTTOM:3000 seems to work
well, but it is probably possible to get away with less.
Example:
________
#include <stdio.h>
#include "sdb.h"
main()
{
DB_SEL *sptr;
char payee[100],amount[100];
/* setup retrieval */
if ((sptr = db_retrieve("checks where amount > 25.00")) == NULL) {
printf("*** error: %s ***\n",db_ertxt(dbv_errcode));
exit();
}
/* bind user variables to attributes */
db_bind(sptr,"checks","payee",payee);
db_bind(sptr,"checks","amount",amount);
/* loop through selection */
while (db_fetch(sptr))
printf("%s\t%s\n",payee,amount);
/* finish selection */
db_done(sptr);
}
SDB - a Simple Database System Page 22
Function:
_________
Setup a tuple retrieval context
Format:
_______
dbptr = db_retrieve(sexpr [ ,arg ]...)
Rules:
______
1. sexpr is a pointer to a string containing an rse
2. arg is a "printf" argument
3. dbptr is a database context pointer
4. db_retrieve returns NULL on errors
5. on errors, the error code is in dbv_errcode
SDB - a Simple Database System Page 23
Function:
_________
Fetch the next set of tuples from a retrieval context
Format:
_______
db_fetch(dbptr)
Rules:
______
1. dbptr is a database context pointer
2. updates the values of all bound user variables
3. db_fetch returns FALSE if no more tuples match or
if an error occurs
4. on errors, the error code is in dbv_errcode
SDB - a Simple Database System Page 24
Function:
_________
Update the current tuple within a retrieval context
Format:
_______
db_update(dbptr)
Rules:
______
1. dbptr is a database context pointer
2. db_update returns FALSE if an error occurs
3. on errors, the error code is in dbv_errcode
SDB - a Simple Database System Page 25
Function:
_________
Store a new tuple within a retrieval context
Format:
_______
db_store(dbptr)
Rules:
______
1. dbptr is a database context pointer
2. db_store returns FALSE if an error occurs
3. on errors, the error code is in dbv_errcode
SDB - a Simple Database System Page 26
Function:
_________
Bind a user variable to the value of a tuple attribute
within a retrieval context
Format:
_______
db_bind(dbptr,rname,aname,value)
Rules:
______
1. dbptr is a database context pointer
2. rname is a pointer to the relation name
3. aname is a pointer to the attribute name
4. value is a pointer to a character array to receive
the attribute value
5. db_bind returns FALSE if an error occurs
6. on errors, the error code is in dbv_errcode
SDB - a Simple Database System Page 27
Function:
_________
Get the value of a tuple attribute within a retrieval
context
Format:
_______
db_get(dbptr,rname,aname,value)
Rules:
______
1. dbptr is a database context pointer
2. rname is a pointer to the relation name
3. aname is a pointer to the attribute name
4. value is a pointer to a character array to receive
the attribute value
5. db_get returns FALSE if an error occurs
6. on errors, the error code is in dbv_errcode
SDB - a Simple Database System Page 28
Function:
_________
Put the value of a tuple attribute within a retrieval
context
Format:
_______
db_put(dbptr,rname,aname,value)
Rules:
______
1. dbptr is a database context pointer
2. rname is a pointer to the relation name
3. aname is a pointer to the attribute name
4. value is a pointer to the new value
5. db_put returns FALSE if an error occurs
6. on errors, the error code is in dbv_errcode
SDB - a Simple Database System Page 29
Function:
_________
Discontinue usage of a retrieval context
Format:
_______
db_done(dbptr)
Rules:
______
1. dbptr is a database context pointer
SDB - a Simple Database System Page 30
Function:
_________
Translate an error code to an error message text
Format:
_______
db_ertxt(errcode)
Rules:
______
1. errcode is an SDB error code
