WEBVTT 00:07.370 --> 00:07.850 All right. 00:07.850 --> 00:12.470 So let me go ahead and show how to delete a foreign key in PostgreSQL. 00:13.130 --> 00:15.860 First of all, let's go ahead and look at our tables again. 00:15.860 --> 00:17.390 So I have this table. 00:17.780 --> 00:23.870 And I see this is a the first of all select all from stress level. 00:24.140 --> 00:32.720 So select all from students and puts semicolon end. 00:32.720 --> 00:36.350 And this is our students table underscore head and down select. 00:39.380 --> 00:44.360 All from foreign which is semicolon end. 00:44.360 --> 00:45.680 And this is what we have. 00:45.860 --> 00:54.290 No the foreign key which we created is a foreign key constraint for the students which references the 00:54.290 --> 00:55.040 table. 00:55.280 --> 00:56.390 Uh form. 00:56.660 --> 00:58.550 Let's go ahead and go back here. 00:58.550 --> 01:01.370 And I'm going to say students. 01:01.370 --> 01:08.270 And uh now you can see that the stiletto has a foreign key constraint named as trace underscore for 01:08.300 --> 01:10.160 id underscore f key. 01:10.190 --> 01:16.910 Why we haven't said f d d and change the font table. 01:16.910 --> 01:26.360 And now this references the students table because the foreign key is actually a constraint in the students 01:26.360 --> 01:26.840 table. 01:26.870 --> 01:29.450 We can see that table student constraint. 01:29.480 --> 01:38.150 Still only the underscore f key foreign key foreign ID references the foreign table which has a permanent 01:38.150 --> 01:40.730 key ID where it references. 01:40.760 --> 01:44.750 Now I can actually go ahead and delete anything from the strange table. 01:44.780 --> 01:45.200 What? 01:45.200 --> 01:50.180 I can delete any D which is assigned to any of the student. 01:50.210 --> 01:51.110 Let's go ahead and see. 01:51.140 --> 01:58.970 I can actually delete uh, Steve and Steve has a this ID three assigned to it. 01:59.060 --> 02:05.420 I can comfortably delete your Steve without affecting the full table was. 02:05.420 --> 02:11.240 I cannot go ahead and delete this Samsung without first deleting Judge Steve. 02:11.480 --> 02:15.710 Let's go ahead and give you an example so it can be able to see what is actually going on here. 02:15.710 --> 02:20.150 So if I go ahead and see uh when I say delete. 02:23.060 --> 02:29.510 So um deletes from from and where. 02:31.790 --> 02:37.730 ID is equal to three and put a semicolon in and hit enter unknown. 02:37.730 --> 02:39.260 You can see we have an error. 02:39.260 --> 02:44.300 It says update or delete on telephone violates foreign key constraints. 02:44.330 --> 02:49.250 Student's phone ID uh f key on table students. 02:49.430 --> 02:55.040 So details key ID equals to three is two the first from table students. 02:55.070 --> 03:01.340 What I can confidently go ahead and delete uh, this table or this row called George which references 03:01.340 --> 03:02.120 that table. 03:02.390 --> 03:05.760 Now let's go ahead and see onto delete. 03:05.790 --> 03:10.290 Go ahead and delete the wrong students. 03:10.290 --> 03:25.290 And the one I'm going to delete is, uh, I'm going to delete from strings where the students where 03:25.290 --> 03:29.220 the students underscore ID is equal to. 03:29.970 --> 03:36.600 This idea of joy and idea of joy is one that is just so where the stress is equal to one. 03:36.630 --> 03:39.660 I put a silicone in and let's go ahead and hit enter. 03:39.660 --> 03:43.560 And down here you can see I have delete one. 03:43.560 --> 03:47.760 So let's go ahead and just select all from. 03:49.590 --> 03:56.430 While I select all from today's table I'll put a semicolon and then hit enter. 03:56.430 --> 04:04.080 And now you can see I hadn't left it a wasteland uh, and took away because we have deleted that. 04:04.080 --> 04:09.150 And the casino definitely is no longer there when you go ahead and select all. 04:11.880 --> 04:17.220 From the Thredbo from file and put all. 04:19.860 --> 04:22.380 For a phone, call it in and hit enter. 04:22.410 --> 04:25.080 You can see we see how some songs are in here. 04:25.140 --> 04:27.420 Now I can comfortably go ahead and delete this. 04:27.420 --> 04:28.080 Some songs. 04:28.110 --> 04:30.150 Let's go ahead and check it out. 04:30.150 --> 04:32.160 So delete one. 04:32.160 --> 04:39.930 So let it phone phone where the ID be equal to three. 04:39.960 --> 04:43.860 And now the score hit and out with a semicolon at the end and hit enter. 04:43.860 --> 04:45.960 And you can see we have delete. 04:45.990 --> 04:49.920 Now let's go ahead and send it over to a phone. 04:49.950 --> 04:55.860 Go ahead and select all from phone up to the end and hit enter. 04:55.860 --> 04:58.560 And you can see that some song has been deleted. 04:58.770 --> 05:04.080 So in this way you can actually, uh, learn how to delete a foreign key. 05:04.110 --> 05:11.220 And you have to also, first of all, delete that, uh, Our table or delete doodle, which reverses 05:11.250 --> 05:12.300 the order table. 05:12.300 --> 05:21.060 And here we have the student table, because we can be able to delete the, uh, any row for the for 05:21.060 --> 05:26.280 the row when it refreshes, or it has a reference from the students table. 05:26.280 --> 05:32.250 So whenever you assign anything to another table, you first have to delete that foreign key before 05:32.250 --> 05:34.500 it can be able to delete the reference table. 05:34.500 --> 05:35.730 So I hope that is cool. 05:35.730 --> 05:41.460 So this is about delete is very important to learn it because you might score head to delete this and 05:41.460 --> 05:45.300 then be like wow, what is going on and how can I be able to delete that? 05:45.300 --> 05:48.150 And you might be like, wow, what is really going on? 05:48.150 --> 05:49.650 How can I be able to do this? 05:49.650 --> 05:51.420 So now we have learned how to do it. 05:51.420 --> 05:55.710 Go ahead and check it out, practice with it and if you have any question or had I used the question 05:55.710 --> 05:59.100 and answer section and I'm going to get back to you as soon as possible. 05:59.130 --> 06:00.000 Thank you so much. 06:00.000 --> 06:03.270 And I'm going to see you in the next video lecture.