Entity-Relationship Model/2 — Transcript

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1. 0:00[Music]
2. 0:15welcome to module 14 of
3. 0:19database management systems
4. 0:21in the last module we started our
5. 0:23discussions on the entity relationship
6. 0:25model
7. 0:26we will continue that in
8. 0:28this module as well and actually
9. 0:31conclude it in the next module so these
10. 0:34are the
11. 0:36items that we had discussed in the last
12. 0:38module
13. 0:39in the present one
14. 0:42we will first illustrate the
15. 0:44entity relationship diagram notation
16. 0:47that graphical notation for
17. 0:50er model how
18. 0:51nicely this can be shown in terms of
19. 0:54certain diagrams
20. 0:55and
21. 0:57then we will explore how er models can
22. 1:00be
23. 1:01translated to relational schemas which
24. 1:04is a basic step
25. 1:06of
26. 1:07the logical design
27. 1:11so these are the
28. 1:13topics so we start with the er diagram
29. 1:17naturally the first thing to represent
30. 1:19in
31. 1:20an er model is the entity set every
32. 1:23entity set is represented by a rectangle
33. 1:26on the top
34. 1:28we write the name of that entity set as
35. 1:30you can see examples here the instructor
36. 1:33and student are the two entity sets
37. 1:35and below that we write
38. 1:37the names of the attributes that are
39. 1:40involved
40. 1:41and we underline the attribute or
41. 1:43attributes that form the primary key of
42. 1:46that entity site
43. 1:50a relationship between ah two entity
44. 1:53sets is represented by a diamond
45. 1:57and two connecting lines to the two
46. 1:59entity sets so here it says that advisor
47. 2:03is a relationship between entity set
48. 2:06instructor and entity set student
49. 2:09trying to
50. 2:10convey the real world situation that
51. 2:14students are advised by
52. 2:17the instructors or students have
53. 2:19instructors and so on
54. 2:24as we had mentioned that relationships
55. 2:27could also have attributes
56. 2:29so if the advisor relationship has an
57. 2:31attribute date
58. 2:33then it will be tagged to the advisor
59. 2:36relationship
60. 2:38with
61. 2:39the attribute coming as a
62. 2:41within a rectangle and attached to the
63. 2:43name of the relationship
64. 2:47by a dotted line
65. 2:49so this shows that advisor is the
66. 2:50relationship between instructor and
67. 2:52student
68. 2:53and the advisor relationship has an
69. 2:55attribute date
70. 2:59it is possible that
71. 3:01the
72. 3:02relationship
73. 3:04that hold between
74. 3:06two entity sets
75. 3:08can be can use
76. 3:11entity sets which are same that is it is
77. 3:13possible that a set is related to itself
78. 3:17so as an example we show the entity set
79. 3:21course
80. 3:22which has a relationship
81. 3:25prereq prerequisite
82. 3:27which takes a course id
83. 3:30and relates it to another course id
84. 3:33called the prerequisite id
85. 3:35because obviously a co if a course has a
86. 3:37prerequisite then that prerequisite
87. 3:40itself is another course id which must
88. 3:42occur in this table itself
89. 3:45so in this case if you can see that
90. 3:48unlike the earlier case ah the
91. 3:51prereq id is not actually a field of
92. 3:53this ah
93. 3:55relation course so we say these are
94. 3:57rules
95. 3:58so we say the role
96. 4:00that prereq relate
97. 4:03from the course
98. 4:06relation to itself
99. 4:09are
100. 4:11course id and
101. 4:13id
102. 4:14where in the actual table both of them
103. 4:17relate to course id but prereq will
104. 4:21pair them to show
105. 4:23which course has what prerequisite
106. 4:28and we often need this kind of we have
107. 4:31seen
108. 4:32ah similar instances of this while we
109. 4:34treated dealt with the recursive queries
110. 4:37in databases the source destination of
111. 4:41ah
112. 4:42airlines problem that we discussed has
113. 4:45similar kind of relationship structure
114. 4:47so you can think about a relationship
115. 4:49flies
116. 4:51from the set of ah
117. 4:54source to this set of destination
118. 4:58and basically this
119. 4:59these two sets the places
120. 5:02source places in the destination place
121. 5:03are necessarily the same set the same
122. 5:06relation
123. 5:08there could be a constraint on the
124. 5:10cardinality so
125. 5:11the line that links the relationship
126. 5:15diamond with the rectangles of the
127. 5:19relations rectangles of the entity sets
128. 5:24those lines could have an arrow at the
129. 5:27end or may not have an arrow at the end
130. 5:30so
131. 5:31if it has got an arrow then it means
132. 5:34that suppose it has an arrow it means
133. 5:37one
134. 5:38and if it does not have a arrow if it is
135. 5:40simple then it means many
136. 5:42so using this notation we can
137. 5:45designate
138. 5:47one to one one to many these all
139. 5:49different kinds of
140. 5:51cardinalities that we had discussed
141. 5:54so for example if we are showing this
142. 5:57ah arrow on both hands both ends of this
143. 6:01relationship advisor then it means that
144. 6:05it is a one to one relationship because
145. 6:07there is an arrow here so this is one
146. 6:11there is an arrow here so this is one
147. 6:14there is an arrow here so this is one
148. 6:17which means that a student is associated
149. 6:19at most
150. 6:21with at most one instructor
151. 6:23and it also means that
152. 6:26an instructor is associated with at most
153. 6:28a student
154. 6:30this may not be a reality this usually
155. 6:31is not the reality but this we are just
156. 6:34showing this as an example
157. 6:36so
158. 6:37if the student
159. 6:39instructor relationship
160. 6:42advisor relationship is one to one
161. 6:45then this is how we will denote it
162. 6:52if it is
163. 6:54one to many
164. 6:57so
165. 6:59which side is one this side is one
166. 7:01and this side is many
167. 7:03so it is one too many from instructor to
168. 7:07student which says that every student
169. 7:10has at most one instructor
170. 7:12and an instructor may have several
171. 7:16students it could be null also it could
172. 7:18be none also
173. 7:20so for an instructor here there are many
174. 7:23students but for a student here there
175. 7:25are only at most one instructor so its
176. 7:28one to many and this is how we designate
177. 7:32a similar thing will happen if i read
178. 7:35the same relation in the other direction
179. 7:39so instructor to student was one to many
180. 7:42so student instructor also drawn in the
181. 7:44same way because this is
182. 7:46ah
183. 7:48this is the one side
184. 7:51this is the one side and this is the
185. 7:52many side so it situation is the same
186. 7:55so
187. 7:56if we read from the student to
188. 7:58instructor then it is also designates
189. 8:00the many to one relationship
190. 8:04and finally we can have a many to many
191. 8:06relationship where there is no arrow at
192. 8:08either end which means that an
193. 8:10instructor is associated with
194. 8:13several possibly none
195. 8:14no student via the advisor relation and
196. 8:17the student may also have several
197. 8:19instructors by the advisor relationship
198. 8:21now you can you can certainly
199. 8:23ah
200. 8:24figure out that in the particular case
201. 8:26of student
202. 8:28instructor scenario of providing advice
203. 8:31one to one as well as many too many are
204. 8:33not
205. 8:34the usual real world scenarios
206. 8:37but ah these are we have just using to
207. 8:41show you how to model this usual
208. 8:43scenario would be from instructor to
209. 8:44student it is one too many relationship
210. 8:51a relationship could be
211. 8:55total or it could be partial
212. 8:57if a
213. 8:58if one side of the relationship or
214. 9:00whichever side of the relationship is
215. 9:03total
216. 9:05then we draw
217. 9:06a double line so you can you can see
218. 9:09here in the diagram we are drawing a
219. 9:10double line which means that in the
220. 9:13advisor relationship the involvement of
221. 9:15the student is total which means that
222. 9:18every student
223. 9:20must feature in the advisor relationship
224. 9:22or in other words every student must
225. 9:25have an advisor
226. 9:27but it is
227. 9:31partial on the
228. 9:35instructor side
229. 9:37because every instructor
230. 9:40may not have a student
231. 9:46so this double line
232. 9:48shows that reality
233. 9:53some entities may not participate in any
234. 9:55relationship is a partial
235. 10:00now
236. 10:01this
237. 10:02constraints the cardinality constraints
238. 10:04can be made more precise by actually
239. 10:07using numbers
240. 10:09you can actually say on the two sides of
241. 10:11the relationship
242. 10:12that at the minimum how many entities
243. 10:14should relate and at the maximum how
244. 10:16many entities can relate
245. 10:19for example if we are saying
246. 10:21that ah
247. 10:24we are on the right hand side here if
248. 10:26you see we are saying
249. 10:27that
250. 10:28it is maximum minimum is 1 maximum is
251. 10:31one
252. 10:32which what does it say it says that
253. 10:35every student
254. 10:37the minimum is one so every student must
255. 10:39feature in the advisor relationship
256. 10:42so
257. 10:44in real world every student must have
258. 10:47a an advisor must have an instructor
259. 10:51it says maximum is one which says that
260. 10:54every student can have at most one
261. 10:56instructor
262. 10:58so this one to one one dot dot one says
263. 11:00that every student must have at least
264. 11:02one instructor every student must have
265. 11:05at most one instructor so together it
266. 11:07says that every student must have
267. 11:09exactly one instructor
268. 11:12whereas if i if you see on this side it
269. 11:15says that 0
270. 11:17dot dot star star stands for no limit
271. 11:22it can be anything any number
272. 11:24so the minimum is 0 which means that an
273. 11:26instructor may not have a student
274. 11:29and star says that it the instructor can
275. 11:31have any number of student
276. 11:34naturally 0 1 2 3 4 2 or 200 so any
277. 11:38instructor can advise any number of
278. 11:40students
279. 11:41so these kind of precise
280. 11:44number constraints
281. 11:46can be put in addition to the one to
282. 11:50many or one to one many to many kind of
283. 11:53notations in the diagram so when we do
284. 11:56that we have the precise cardinality of
285. 11:58the complex relations that exist
286. 12:11next we take a look into the handling of
287. 12:14the complex attributes
288. 12:16the first you remember that first kind
289. 12:19of complex attribute is one which is
290. 12:21composite
291. 12:22say name which has first name middle
292. 12:24name
293. 12:25initial
294. 12:27middle initials and
295. 12:29last name
296. 12:36so
297. 12:38when we have that then
298. 12:42the way we represent is
299. 12:44at the
300. 12:46actual name of the attribute is at the
301. 12:48outermost level
302. 12:50and its
303. 12:52composites are written with
304. 12:55certain
305. 12:56shift on the left so these all say that
306. 12:58these are composites of name
307. 13:01so it this says that street city state
308. 13:04zip are composites of address
309. 13:07and further indentation say that these
310. 13:10are composites of straight
311. 13:12so
312. 13:13this is how graphically we show
313. 13:15that
314. 13:16how complex attributes feature
315. 13:24now
316. 13:25let us go back to discussing the weak
317. 13:27entity sets
318. 13:29in the ear diagram a weak entity set
319. 13:32is represented by a double
320. 13:36rectangle you remember the section is a
321. 13:38weak entity set and why is it so
322. 13:40because a same course may have
323. 13:44two different i am sorry two different
324. 13:46courses may have the same section id
325. 13:49semester and year that is two courses
326. 13:51two or more courses
327. 13:53may run
328. 13:54sections by the same name in the same
329. 13:56semester and the year
330. 13:58so a section cannot be uniquely
331. 14:00identified by these three
332. 14:03attributes
333. 14:04it needs a relationship
334. 14:07with the identifying entity set course
335. 14:12to be specific the course id
336. 14:14so that the entities here in can be
337. 14:17uniquely specified
338. 14:19so since this has happened so we
339. 14:21designate that by putting this
340. 14:25double rectangle around the weak entity
341. 14:28set section
342. 14:32we underline the discriminator of a weak
343. 14:34entity set with dashed line so you
344. 14:37remember these are the discriminators
345. 14:40because given the identifying
346. 14:44attribute in the identifying set
347. 14:46these are the
348. 14:48attributes which distinguish
349. 14:51different tuples of section
350. 14:54so
351. 14:55they are not shown
352. 14:56with solid underline they are shown as
353. 14:59dotted underline dashed underlined so
354. 15:02that you can make out that this is the
355. 15:03weak entity set and these are the
356. 15:05discriminators
357. 15:10the relationship set connecting the weak
358. 15:12entity set to the identifying strong
359. 15:14entity set is also
360. 15:16so this the moment you have weak entity
361. 15:18set you know that there has to be a
362. 15:20relationship
363. 15:21to the strong entity set which
364. 15:23identifies it so that relationship sec
365. 15:26course which say
366. 15:28course id against this
367. 15:31binds that
368. 15:32is
369. 15:33designated with a double diamond so that
370. 15:36you know that this is
371. 15:38the
372. 15:38identifying relationship
373. 15:41between a weak entity set and the
374. 15:43corresponding strong entity set
375. 15:48and once that happens then the primary
376. 15:50key becomes
377. 15:52the discriminators of section the weak
378. 15:55entity set and the primary key of the
379. 15:59identifying a strong entity set the
380. 16:02course
381. 16:03so that forms our
382. 16:06final
383. 16:07primary key for this entity set section
384. 16:10mind your course id is not
385. 16:12a part of
386. 16:14this relation but it actually plays the
387. 16:16role
388. 16:17through this section id
389. 16:19as a key for the section
390. 16:22relation without which the section
391. 16:25entities in the section cannot be
392. 16:27uniquely identified
393. 16:30so having said that this is a the er
394. 16:33diagram of the university enterprise
395. 16:36some of the points that you could take a
396. 16:38look at
397. 16:39this is the weak entity set we have just
398. 16:41seen this is the
399. 16:44relationship to the identifying strong
400. 16:46entity set
401. 16:49this is a prerequisite
402. 16:51multi role
403. 16:52relationship
404. 16:54ah this you can see is a is a total
405. 16:59involvement
406. 17:00so why is it a total involvement because
407. 17:02every section must have at least
408. 17:05one teacher so there cannot be a section
409. 17:08which does not feature in the teachers
410. 17:10relationship
411. 17:11similarly every section must get a time
412. 17:14slot
413. 17:15where
414. 17:16the classes for that section is held
415. 17:18so every section must feature in the sec
416. 17:21time slot so these are the
417. 17:23similarly it must get a class room so
418. 17:26these are all different
419. 17:28total ah
420. 17:31involvements that we total roles that
421. 17:33you can see we can see some of that
422. 17:34elsewhere as well
423. 17:36for example you can see it here we can
424. 17:38see it here
425. 17:39because in between instructor
426. 17:42and the department the ins department
427. 17:45relationship
428. 17:46certainly every instructor must have a
429. 17:48department so it is total
430. 17:50but it is not the same for the
431. 17:52department every department will not
432. 17:54have instructors
433. 17:56so
434. 17:57this is how if we can you can go through
435. 18:00carefully and
436. 18:01for example this is another which is
437. 18:03total which means that every course
438. 18:06need a department you cannot run a
439. 18:08course which does not have a department
440. 18:11so
441. 18:12this is how we can see that how the er
442. 18:15diagram the first conceptual level
443. 18:17diagram of a very simple university
444. 18:20enterprise is being designed
445. 18:22following the notions and symbols of er
446. 18:26model that we have already developed
447. 18:30next comes ah the part where from this
448. 18:33model which is primarily diagram based
449. 18:35we have to really go to the relational
450. 18:37schema which is names of relations and
451. 18:40attributes which is
452. 18:41ah pretty much a straight forward job so
453. 18:44entity sets and relationship sets have
454. 18:46to be represented in terms of relational
455. 18:49schema what is the beauty of the
456. 18:52er model and the relational schema is
457. 18:54that that when you reduce the entity
458. 18:57relationship model to relational schema
459. 18:59both entity sets and relationships
460. 19:02sets both of them turn out to be
461. 19:04relational schemas
462. 19:06so that the database finally can be
463. 19:08represented simply as a set of
464. 19:11schemas each one of which must have a
465. 19:15set of identifying primary key
466. 19:19so let us
467. 19:22look into that so on the
468. 19:25strong entity set that reduces to schema
469. 19:28with the same attributes is student so
470. 19:31student has id
471. 19:33name and total credit
472. 19:35so which we
473. 19:37saw earlier now this gets converted to a
474. 19:39schema with id being the
475. 19:43primary key
476. 19:45the other case of weak entity set
477. 19:47section
478. 19:48which had three discriminators and true
479. 19:51sec course relationship was
480. 19:54identified from the strong entity set
481. 19:57course
482. 19:58borrows the
483. 20:00primary key of the course to be defined
484. 20:03in terms of this
485. 20:05relational schema
486. 20:08one moment
487. 20:11this borrows
488. 20:13the primary key from here
489. 20:15and becomes
490. 20:17so you can see that
491. 20:19in the er model
492. 20:21the section did not have
493. 20:23course id as an attribute but
494. 20:26while we reduce this to the relational
495. 20:29schema
496. 20:30through this sec course relationship we
497. 20:33have borrowed this primary key from
498. 20:35course
499. 20:36the primary key of course
500. 20:38the course id
501. 20:40and added that to section to make it a
502. 20:42complete relational
503. 20:44schema
504. 20:49next comes the representation of
505. 20:51relationships so we are showing a
506. 20:53relationship advisor so which relates
507. 20:56instructors to students so naturally
508. 20:59every instructor is identified by id
509. 21:02every student is identified by id
510. 21:05since both of the attributes have the
511. 21:07same name id we are
512. 21:09calling them as s underscore id and for
513. 21:11the student and i underscore id for the
514. 21:13instructor so the advisor relation is
515. 21:17basically
516. 21:18a pairing of these two ids
517. 21:21which gives rise to a relationship
518. 21:24which looks like this relationship
519. 21:26schema which looks like this so we can
520. 21:28in general say that if we have a
521. 21:29relationship ah
522. 21:31in the er model which we want to
523. 21:33represent in the
524. 21:35schema then we will take
525. 21:38we will create a schema which has the
526. 21:42primary key of both the sets and put
527. 21:44them together and if the names clash we
528. 21:47will just change the name with the
529. 21:50name of the relation and that will give
530. 21:53us the schema for the relationship
531. 21:56in this case the advisor
532. 21:58so we have seen how to represent entity
533. 22:00sets weak entity sets and
534. 22:03relationships ah let us look at how do
535. 22:05we deal with composite attributes
536. 22:07because ah
537. 22:09so far we had assumed that
538. 22:11the relational schema has attributes and
539. 22:14every attribute has a domain
540. 22:17ah the type from where its values come
541. 22:19so if i have a composite attribute where
542. 22:22every attribute has a set of components
543. 22:24then the easiest way to handle this is
544. 22:26to what is known as flatten
545. 22:29the composite attribute so flattening
546. 22:31basically is for example if i take a
547. 22:34name it has three
548. 22:36components so each one of them i can
549. 22:39call by
550. 22:40given new name name underscore first
551. 22:43name name underscore middle initial name
552. 22:45underscore last name
553. 22:47by prefixing with the attribute name
554. 22:51i make the names of these components
555. 22:53necessarily unique
556. 22:55now
557. 22:56after i do the prefixing i might figure
558. 22:59out that actually prefixing is not
559. 23:01required first name itself is a unique
560. 23:03because it does not occur anywhere else
561. 23:05if it is then i can i may drop the
562. 23:08prefix name but in general i can take
563. 23:11the attribute name prefix on the
564. 23:13component and just flatten them out make
565. 23:15them all attributes each separate
566. 23:18attribute so
567. 23:20here when we ah flatten out we will have
568. 23:24ah first name
569. 23:26middle initial last name as you can see
570. 23:29these
571. 23:32flattened out from here
572. 23:35then we have street number
573. 23:37street name
574. 23:38apartment number flattened out from the
575. 23:41street
576. 23:42subsequently we have city state zip
577. 23:45flattened out from here so all of them
578. 23:47flattened out has become separate
579. 23:49attributes and flattening is a very
580. 23:52straightforward mechanism by which you
581. 23:54can convert complex composite attributes
582. 23:57into the
583. 23:58regular schema design
584. 24:02you get into little bit of issue if you
585. 24:04have multi valued attribute multivalued
586. 24:06attribute is one where one attribute may
587. 24:09have multiple values at the same time
588. 24:11and the example we talked about
589. 24:14is ah
590. 24:15a phone number i may have multiple phone
591. 24:17numbers
592. 24:18so certainly against an attribute i can
593. 24:21keep only one value
594. 24:23so if i if my attribute is multiple
595. 24:25value then the basic idea is to use a
596. 24:27separate schema to maintain this
597. 24:30multiple values for example if i have to
598. 24:32maintain multiple
599. 24:34phone numbers of an instructor i may
600. 24:36have a separate i may decide to have a
601. 24:38separate relation
602. 24:40which relates the key of the
603. 24:42instructor relation
604. 24:44and
605. 24:45the attribute that i want to
606. 24:47ah maintain multiply so in this
607. 24:50relationship in this relation
608. 24:52inst
609. 24:53underscore phone against the same id
610. 24:56i can have
611. 24:57different phone numbers so there will be
612. 24:58different records which match on the id
613. 25:01but do not match on the phone number
614. 25:03which gives me the different values that
615. 25:05the phone number can take
616. 25:07and then
617. 25:08this inst phone in conjunction with the
618. 25:10instructor relation will actually denote
619. 25:15the
620. 25:15multivalued
621. 25:17phone number attribute so this is just
622. 25:20an example showing that
623. 25:22for one primary key of an instructor
624. 25:26ah one
625. 25:27two two two two two
626. 25:29and there are two phone numbers so this
627. 25:31will basically mean you have two tuples
628. 25:33in the new relation
629. 25:35so with that we can ah handle multiple
630. 25:38multi valued attributes also
631. 25:42some of the relationships that we may
632. 25:44have modeled which we have done
633. 25:47in
634. 25:48doing the
635. 25:50database er schema
636. 25:52could have redundancy for example take a
637. 25:55case here we have the instructor
638. 25:58and we have the student the advisor
639. 26:00relation is incidental here
640. 26:02and we have department
641. 26:04so we want to say that the instructors
642. 26:06belong to
643. 26:07certain departments every instructor
644. 26:09belongs to one department which is the
645. 26:11totality of the relationship here
646. 26:13similarly every student belongs to a
647. 26:15department totality of the relationship
648. 26:16on this side
649. 26:18and inst debt
650. 26:20ins debt in that context is a
651. 26:22relationship
652. 26:23which is between instructor and
653. 26:26department similarly
654. 26:31so we can
655. 26:33there is certain redundancy in this
656. 26:35because
657. 26:36we can get make this simpler if we just
658. 26:39take the
659. 26:42primary key of this relation and put in
660. 26:44here
661. 26:47if we do that then basically this become
662. 26:50redundant these are normal required so
663. 26:52all that you are saying is the
664. 26:54instructor has a depth name field which
665. 26:57says which department does it belong to
666. 27:00so if there is a choice between whether
667. 27:02you will keep such
668. 27:04relationships or you will
669. 27:07actually
670. 27:08reduce the redundancy in the schema and
671. 27:11involve the
672. 27:12primary key of the other relation into
673. 27:16your
674. 27:17primary table which is instructor or the
675. 27:20student here
676. 27:23so instead instead of creating a schema
677. 27:25for relationships at ins department
678. 27:27you will simply add a department name
679. 27:30so mind you this is at the er model
680. 27:32level you did have a separate
681. 27:34relationship but while you reduce it to
682. 27:38your relationship relational schema you
683. 27:41are reducing that relationship by
684. 27:44including the depth name as an attribute
685. 27:47in instructor so that is called the
686. 27:49reduction of schema which is often used
687. 27:54ah for one to one relationship so this
688. 27:56is this was ah this is a good if you
689. 28:00have many to one relation because this
690. 28:02was possible because
691. 28:04every instructor has one department so
692. 28:07if you just include the department name
693. 28:09with the instructor or every student has
694. 28:12one department
695. 28:14ah so it is possible that way but if you
696. 28:17have a one to one relationship
697. 28:19then naturally
698. 28:21you can do the similar reduction
699. 28:23by i by choosing the either side
700. 28:26as as the many you know because because
701. 28:28the the unique side has to come on the
702. 28:30many side the unique side here this is
703. 28:33the unique side here
704. 28:35because
705. 28:36every instructor has a unique department
706. 28:38and this is the many side here so the
707. 28:40unique side has to attribute has to come
708. 28:42in here the unique side primary key has
709. 28:44to come in here so instead of ah
710. 28:49so you can
711. 28:50apply the same principle to a one to one
712. 28:52relationship by treating any one of them
713. 28:54as a many side
714. 28:56and add the extra attribute on the other
715. 28:58side to get rid of this additional
716. 29:01schema the schema corresponding to a
717. 29:03relationship set linking a weak entity
718. 29:06set to its underlying strong entity set
719. 29:08is certainly redundant we have already
720. 29:11sent this
721. 29:12so this ah
722. 29:15is made redundant by including
723. 29:18the
724. 29:19primary key of the identifying relation
725. 29:23identifying entity set in the weak
726. 29:25entity set
727. 29:27so that is another reduction
728. 29:29of schema that can be
729. 29:31done
730. 29:32so to summarize ah
731. 29:35we have in this module
732. 29:37illustrated the entity relationship
733. 29:40diagram which are very nice ways of
734. 29:43graphically representing what we see in
735. 29:46the
736. 29:47real world
737. 29:49so it has graphical representation of
738. 29:51entity set
739. 29:53attributes
740. 29:54the key attributes primary key
741. 29:56attributes
742. 29:58the weak entity sets
743. 30:00and
744. 30:01the relationships
745. 30:05along with the cardinality information
746. 30:08and then we have shown that using
747. 30:11certain reduction rules
748. 30:14how we can easily reduce this
749. 30:17entity relationship diagram or entity
750. 30:19relationship model
751. 30:22into the
752. 30:23traditional
753. 30:24relational schema
754. 30:26and we have seen that both the entity
755. 30:29sets
756. 30:30as well as the relationship sets become
757. 30:33relational schemas