1 CS 430 Database Theory Winter 2005 Lecture 13: SQL DML - Modifying Data

2 SELECT Three statements for modifying data  INSERT - Add rows to a table  UPDATE - Update rows in a table  DELETE - Delete rows in a table

3 INSERT First form:  INSERT INTO [(, … )] VALUES ( );  Creates a new row in with the specified values for the specified columns  Column list is optional, if omitted, the column order from CREATE TABLE is used (same as SELECT.*) Not recommended.  Values can be an explicit value or the key word DEFAULT

4 INSERT, Form Two Second form:  INSERT INTO SET =, =, …;  Sets the specified columns to the results of the specified expressions  Expressions can refer to the values set in previous expressions  Expressions can be the keyword DEFAULT

5 INSERT, Form Three Third form:  INSERT INTO [(, …, )} SELECT …;  Inserts multiple rows into a table As many as returned from SELECT  Matches the columns in in order with the columns in the SELECT statement  If no columns are given for uses the column order from the CREATE TABLE

6 Creating a [Temporary] Table Approach 1:  CREATE TEMPORARY TABLE ( );  INSERT INTO SELECT … ; Approach 2:  CREATE TEMPORARY TABLE SELECT … ; Creates a temporary populated as per the SELECT statements Definitions as part of the table creation are optional Table column names are taken from SELECT (may need to create synonyms for column names)

7 UPDATE SET =, … WHERE ; Modify the rows specified in the where condition Modify the columns as indicated  Column names appearing in expressions are modified to their most recent values Could be either the current value in the table or a value in an expression

8 Notes on Update The following is legal:  UPDATE SET age = 2 * age, age = age + 1; Same as age = 2 * age + 1 Complicated WHERE clauses usually require subqueries

9 More notes on update: MySQL specific syntax:  UPDATE, SET = … WHERE ;  Can modify multiple tables simultaneously  Use. where needed  Can be used to substitute for a subquery

10 DELETE DELETE FROM WHERE ;  Delete the matching rows from  “DELETE FROM ;” Will empty table MySQL:  DELETE FROM, …, USING, …,,, …, WHERE ; Delete from multiple rows simultaneously

11 Examples

12 Making Big Changes to the Database Typical Scenario:  Applications are changing As a result required structure of database needs to change  Need to reorganize the database Approach:  Create new tables with new structures  Create new content in new tables Select from old table, or Select to flat file, modify flat file, Load data back to file  Drop old tables and rename new tables

13 More on Making Big Changes And as always:  Test, test, test  Try to do it in small steps And when you’re going to do it …  Take a weekend  Make a backup  Make the changes  Test the changes  Either restore the backup or go live And if you have to do it live  Plan it in incremental steps changing applications and data together  Design applications to use both old and new data