Database Modelling. Lecture 5 Part 1: Updating Database 1/6/2015 1

Transcription

1 Database Modelling Lecture 5 Part 1: Updating Database 1/6/2015 1

2 Learning Objectives 1. To consider how to do insertions and deletions in SQL 2. To consider amendments (updates) to a relation 3. To consider making changes permanent 1/6/2015 2

3 Updating a Relation Having created a relation, we need to be able to : INSERT tuples into it, DELETE tuples from it, and UPDATE tuples in it. It is also essential that, having changed one or more tuples in a relation, the changed value of the relation still satisfies all the integrity constraints. 1/6/2015 3

4 Example of Insertion Value of the EMPLOYEE relation before the insertion. We need to insert these two tuples to the EMPLOYEE relation Value of the EMPLOYEE relation after the insertion. 1/6/2015 4

5 SQL : Insert There are 2 forms of SQL syntax To insert a tuple literal - i.e. a tuple all of whose values are literally written out - the first syntactical form is used. To insert the value of a named relation or any SQL expression, the second syntactical form is used. 1/6/2015 5

6 Layout of Statements in SQL The first (key)word in a statement indicates what kind of action is to be carried out: Select indicates that a retrieval on the DB is to be carried out Insert indicates that an insertion into a relation is to be executed. Delete indicates that tuples are to be removed from a relation. 1/6/2015 6

7 Examples of SQL Insert 1. Insert two literal tuples, i.e. the earlier graphical example Insert Into EMPLOYEE Values ( E2, Nash, M, ) ; Insert Into EMPLOYEE Values ( E4, Archer, S, ) ; 2. Insert the contents of relation EMP into EMPLOYEE Insert Into EMPLOYEE Select * From EMP ; SQL permits only one tuple literal to be entered at a time. So two insertions have to be made. The only limitation on inserting literal tuples is that only one can be entered at a time. There is no limit to the number of tuples that the retrieval can provide for insertion. 1/6/2015 7

8 Insertion : Default Attribute Order Although a tuple is a set of attributes, the attribute values have to be written out in some physical order Ideally match is made on attribute name so that the physical order does not matter. So far we have used the default sequence the sequence in which the attributes appeared in the Create Table statement. If the attribute values are written out in the same physical sequence, then they will be matched by position. 1/6/2015 8

9 SQL : Insert with Attribute Order The 2 revised forms of SQL syntax are Insert Into RELATION_NAME ( names of all the attributes in sequence ) Values ( a value for each attribute in one tuple ) ; Insert Into RELATION_NAME ( names of all the attributes in sequence ) Select.. From.. 1/6/2015 9

10 Further Examples of SQL Insert 1. Insert 2 literal tuples, i.e. the earlier graphical example 2. Insert the contents of relation EMP into EMPLOYEE : Insert I nto EMPLOYEE ( EName, M_ S, Sal, ENo ) Select EName, M_ S, Sal, ENo From EMP ; Any attribute sequence can be used (as names available) 1/6/

11 SQL : Insertions Involving NULLs Use the Insert statement without an attribute name sequence parameter; use the keyword NULL for the relevant attribute(s). 1/6/

12 Effect of Not Null Constraint Because a complete tuple must be inserted any attributes omitted from the insertion will receive nulls by default in order to complete the tuple. If any attribute is prohibited from receiving nulls by its integrity constraint an error will result if the insertion tries to enter null for that attribute. 1/6/

13 SQL : Delete Delete From RELATION_NAME Where condition ; In the Where phrase, a condition is written that picks out the required tuples from RELATION_NAME. These tuples are then deleted from RELATION_NAME. The Where phrase is optional. One can just write Delete From RELATION_NAME ; In this case, all the tuples in RELATION_NAME are deleted from it (but RELATION_NAME still exists, albeit empty). 1/6/

14 Examples of SQL Delete 1. Delete two tuples, i.e. the earlier graphical example. This will be done by specifying tuples where the salary is less than 30,000 Delete From EMPLOYEE Where Sal < ; 2.Delete all the tuples in EMPLOYEE Delete From EMPLOYEE ; Relation EMPLOYEE is now empty. 3. Delete a specific tuple, i.e. the one referring to employee E1 : Delete From EMPLOYEE Where ENo = E1 ; Because ENo is a candidate key, we can be sure that we have deleted one specific tuple, which will correspond to the right person. 1/6/

15 Example of Amendment Value of the EMPLOYEE relation before the amendment. We need to give a pay rise of 500 plus 10% of the current salary: Sal = Sal * 1.10 Value of the EMPLOYEE relation after the amendment. 1/6/

16 SQL : Amendments In SQL, update is given the more specific meaning of amend. Update RELATION_NAME Set attribute assignment(s) Where condition ; In the Where phrase, a condition is written that selects the subset of required tuples from RELATION_NAME. These tuples are then amended. The Where phrase is optional. One can just write Update RELATION_NAME Set attribute assignment(s) ; In this case, all the tuples in RELATION_NAME are amended. 1/6/

17 SQL Update Details Update indicates that an amendment is to be carried out. Set phrase contains the attribute amendment assignments (using = as the assignment symbol) the following Where phrase specifies which tuples of the relation are to be amended omission of Where phrase specifies that all tuples in the relation are to be amended. Set here means set the column(s) to 1/6/

18 Examples of SQL Amendments 1. Amend all the tuples to give everyone a salary increase of 10% plus 500; i.e. the earlier graphical example Update EMPLOYEE Set Sal = ( 1.1 * Sal ) ; 2. Amend Mitchell s name to Wilson, give him/her a new salary of 27,000, and amend his/her marital status to widowed : Update EMPLOYEE Set Sal = 27000, EName = Wilson, M_S = W ; Note how the Where phrase (or lack of it) determines which tuples are amended. 1/6/

19 Further Example of SQL Amend Comparing this with the earlier EMPLOYEE relation, only one tuple in it has now been amended. Update EMPLOYEE Set Sal = 27000, EName = Wilson, M_S = W Where ENo = E5 ; To be certain of amending one specific tuple, a candidate key must be used in the condition. 1/6/

20 Setting Values to Null It is also possible to amend an attribute so that it becomes null instead of containing a value. For example Update EMPLOYEE Set Sal = NULL Where ENo = E5 ; = is still used for value assignment, even though no value is actually being assigned. NULL is a keyword here; so it can be in upper or lower case as long as it is not in speech marks. This sets the value of E5 s salary to be null. Update EMPLOYEE Set Sal = NULL ; This sets everyone s salary to be null! 1/6/

21 Integrity Constraints A tuple to be inserted into a relation must meet all the integrity constraints for that relation. If it does not, it is rejected with an error message. An amended version of a tuple must meet all the integrity constraints for that relation. If it does not, it is rejected with an error message. The deletion of a tuple can be rejected if the deletion breaks an integrity constraint, e.g. a foreign key relationship. 1/6/

22 Making Changes to DB Permanent After using Insert, Delete or Update, you should make sure that the changes made are applied correctly (using SELECT). Once ready to make the changes permanent, use: COMMIT; Commit will write all the changes sinse the last commit to the database. If you are not happy with the changes, then do not use Commit, instead use: ROLLBACK; Rollback will undo all the changes since the last commit. 1/6/