Copyright © Curt Hill SQL The Data Manipulation Language

Copyright © Curt Hill Data Manipulation Language The DML is used for most tasks Generate queries – Covered in previous presentations Insert records Delete records Update records All the tables must exist prior to any of these

Copyright © Curt Hill Use Statement in SQL Server The use changes the database that the next statements will act upon The server may have several databases –This selects which one to use –Leaving it out usually lets the rest of the commands apply to the database named master Example: Use college

Copyright © Curt Hill Insert Form Insert one record into a table General form: INSERT INTO table (f1,f2,…fn) VALUES (v1,…vn) Table is the relation to insert a new record into The fn values are the field names The v1 values are the values to make up the record If a field and its value are left out a null is inserted

Copyright © Curt Hill More on Insert The field names and values are paired –The Nth name receives the nth value The field names may be left out if every column receives a value –The values then match the order given in the Create Table All the values are literals –Character constants in quotes –Numbers appear as themselves All constraints are checked

Copyright © Curt Hill Second Insert Example Insert Into course (number,crhr,title, dept) VALUES (141, 4, 'College Physics‘, 'PHYS') Insert Into course VALUES ('PHYS', 141, 4, 'College Physics')

Copyright © Curt Hill Update Change values in one or more records General form: UPDATE table_name SET field_name = constant, fn2 = c2, … WHERE condition Like the Select the Where is optional –Leaving it out updates every record –A similar thing can be done with the default when creating table

Copyright © Curt Hill Update Examples Update students set s_balance = 0 –Sets all student balances to zero UPDATE course SET c_title = “Dumb course” WHERE dept = ‘CS’ AND number = 170

Commentary The database may restrict updates that violate constraints whether they would cause problems or not Clearly we cannot update a primary key into another one that exists Updates that violate foreign keys or other constraints will also be terminated If the change is to keys, better to delete and then insert Copyright © Curt Hill

Delete Delete one or more records Similar form to a Select Form: DELETE FROM table_name WHERE condition Any records that match are removed Always safest if you match the primary key or do a similar query

Copyright © Curt Hill Delete Example DELETE FROM course WHERE dept = ‘CS’ AND number = 170 DELETE FROM faculty WHERE naid = 144

Copyright © Curt Hill Cascaded Deletions Insertions may only be allowed if the constraints are honored A deletion may change constraints as well In the case of deleting a student –All the grades for that student must also be deleted Different databases may handle the cascade problem differently

Example delete Delete a student: delete from students where s_id = 1184 Disallowed unless grades are first deleted: delete from grades where g_naid = 1184 Copyright © Curt Hill

Allowing cascaded deletes The default behavior is to disallow any insert/update/delete that violates integrity There is no getting around inserts Updates and deletes can allow cascading, but it must be specified with the Create Table or Alter commands The ON clause is attached to the foreign key clause Copyright © Curt Hill

Allowing Cascade Deletes Specify: on delete cascade as part of the foreign key declaration Example: Create Table grades ( … constraint g_fk2 Foreign Key (g_naid) References students (s_id) On delete cascade, … ) Could be update as well Copyright © Curt Hill

Into Clause A Select creates a relation The relation is generally used and then discarded You may also create a new permanent relation with the INTO clause This goes between Select and From Usually used for tables needed for a short period of time –Minutes to days

Copyright © Curt Hill Into Example Students taking CS classes: Select s_id, s_name, s_address INTO CS_Students From students, grades Where g_naid = s_id AND g_dept = 'CS'

Into Commentary This does not need the field definitions like a create table –They are copied from original fields A different field name can be obtained by using an alias A similar thing can be done with a view, but a view does not require storage Copyright © Curt Hill

Later Updates A table created with an Into is now a separate table with separate data A view has no real data –The data belongs to the queried tables When the original table is updated: –The View is changed –The Into table stays the same The Into table may also be updated Copyright © Curt Hill