There are more terms I want to talk to you about. One of those is commit, or two of those are commit and rollback. In Oracle, commit and rollback, specify if you want to save data to the table permanently or if you want to undo or roll back your changes. Let me show you right over here. If I do a select star from employees, which is my table, notice I get three rows, John Adams, James Monroe, Serena Williams. If I happen to insert, insert into employees and I say values and I say four comma, John Williams and I say my department is three, let's say. I'm going to insert this. I just inserted it, then I do a select star from employees. I run this, notice I get four John Williams right here. Because that's the last entry that I made with a department ID of three. But I have not committed this yet. If I do a rollback like this, and I run this, look, it says rollback complete. Now, if I do a select star from employees, I need to spell that correctly. I run this notice, it goes back to three rows, it dropped John Williams. However, if I do a insert into employees and I say values for John Williams and three. Then I do this, and then I look at what the result is, select star from employees. I see that I've got John Williams right here, and I issue a commit like this that's saved it to the table. If I do a select star from employees like this, and you notice it shows me John Williams. If I do a rollback now, it will roll back to the last commit. My last commit means that I have already saved John Williams, which means that it is not going to go back. There you go. Commit will allow you to save your changes onto the table. Rollback, will roll back the changes up until the last commit. Commit and rollback. Constraint, constraints are limitation you place on table or columns within a table. Primary key is a constraint, foreign key is a constraint, unique is a constraint, not null is a constraint. When I look at the department table, notice that I have nullable null, these are constraints right here. In other words, just basically saying that department id and department name cannot contain null values. Go to sales. Notice it says product ID, this can be null, but sales ID cannot. Constraints, primary key is a constraint, foreign key is a constraint, unique is a constraint, non-null is a constraint, and many others. CRUD, and sometimes CRUDS. They're really talking about create read, update, delete or create read, update, delete and search. These are usually represented when you're talking about application, application development, or the type of application that you are doing, or the type of task you're performing. You may say I have a CRUD or CRUDS application. I'm going to be writing a CRUDS application. That means that I'm going to write an application that's going to allow for creation, reading from creating new rows, reading from updating existing rows, deleting rows and so on from a database CRUD or CRUDS. Dual. Dual is a dummy table. This is difficult to understand unless you see what I mean. It is really used to complete the Oracle SQL syntax. Let me show you what I mean. If I say select user, like that, this is not complete syntax. It's going to give me an error because it doesn't know what that is, is missing the from keyword. You see how it says, from keyword is not found where it's expected. I'm just going to do from Dual and I'm going to do a run this and I'll say the user's admin. Dual is a table in Oracle that has nothing in it. If I do a describe Dual which is, let me describe the table, notice it has a column called dummy. The datatype is VARCHAR2 with one character. It's really not used to insert or delete anything, it's just used to complete the syntax. Dual special table, Dummy table is used to complete the SQL syntax.
