IS2803 Developing Multimedia Applications for Business (Part 2) Lecture 5: SQL I Rob Gleasure robgleasure.com

IS2803 Today's lecture  The SQL DML

Introduction to SQL So once we have our data models in place and our DBMS set up, how do we get started using our data?  The SQL (pronounced like sequel) query language SQL (Structured Query Language) was introduced in the 70’s and has evolved since to add features such as  Compatibility with object-oriented programming  Compatibility with XML  Compatibility with XQuery

Accessing a DBMS with SQL

The Structure of SQL SQL can be divided into two parts:  The Data Manipulation Language (DML) Used to create, read, update and delete tuples (CRUD operations) Mostly used by application programmers and sophisticated/specialised users  The Data Definition Language (DDL) Used to define database structure (relation schemas) and data access control, as well as integrity constraints and views Mostly used by database administrator

Basic SQL query The basic syntax of SQL queries are as follows: COMMAND column_name1, column_name2, … FROM/SET/VALUES table_name1, table_name2, … WHERE column_name comparison_operator value; All statements in a semi-colon

The SQL Select statement The SELECT statement is used to retrieve data from a database into a result table, or result-set You can use an asterisk (*) instead of a column name if you wish to select all columns satisfying the criteria e.g. SELECT * FROM my_table;

The SQL Select statement Let’s open up an online example from W3Schools 

The SQL Select statement Sometimes non-primary key columns may contain duplicate values - you can also use SELECT DISTINCT when you want to avoid duplicates in your result-set e.g. SELECT DISTINCT * FROM my_table;

The SQL Where clause A number of comparison operators are possible with the WHERE clause Examples  p?filename=trysql_select_between_in  p?filename=trysql_select_in OperatorDescription = Equal <>, != Not equal > Greater than < Less than >= Greater than or equal <= Less than or equal BETWEEN Between an range of numbers LIKE Searches for patterns IN When a specific value is sought

The ‘LIKE’ condition and Wildcards Sometimes we want to identify records based on slices of the data within a cell - we do this using wildcards and the LIKE condition Examples    WildcardDescription %A substitute for zero or more characters _A substitute for a single character [charlist]Sets and ranges of characters to match [^charlist] or [!charlist] Matches only a character NOT specified within the brackets

The SQL Where clause (continued) Numerous clauses can be combined with the keywords AND & OR e.g. SELECT * FROM my_table WHERE val1=‘true’ AND val2=‘false’; Complex clause can be creates by nesting clauses in parentheses e.g. SELECT * FROM my_table WHERE val1=‘true’ AND (val2=‘false’ OR val2=‘null’); Example 

The SQL Order By keyword The ORDER BY keyword is used to sort the result-set e.g. SELECT * FROM my_table ORDER BY NAME; You may also specify whether you want the returned result-set to be sorted in ascending or descending order by using the keywords ASC or DESC e.g. SELECT * FROM my_table ORDER BY NAME DESC; Example 

More on the SQL DML So far we’ve looked at getting data from specific tables, but there are still two parts of the Data Manipulation Language (DML) we haven’t covered The DML has four main elements  Select  Insert Into  Update  Delete Also, what if our data isn’t on one table?…

Insert Into We use INSERT INTO queries to add new tuples (aka records, rows) into a table The basic structure of an INSERT-INTO query is as follows INSERT INTO table_name1 (column_name1, column_name2, …) VALUES value1, value2, …;

Insert Into For example, say we want to insert a new student in a Students table with a Student_ID of and a Name of “Jane Smith”, we might have the following INSERT INTO Students (Student_ID, Name) VALUES ( , “Jane Smith”) ; Note that we insert the data in the form of a new tuple (aka record, row) and if we do not specify a column, we have to provide data for each column in the new record Example

Update We use UPDATE queries to modify existing tuples (aka records, rows) in a table The basic structure of an UPDATE query is as follows UPDATE table_name1, table_name2, … SET col_name1 = value1, col_name2 = value2, …; WHERE some_column=some_value;

Update For example, say we want to change the previously added student record to be “Janet Smith”, we might have the following UPDATE Students SET (Name = “Janet Smith”) WHERE (Student_ID= ) ; Careful with this, if you don’t set a WHERE condition you will change every record in the database Example

Delete We use DELETE queries to remove existing tuples (aka records, rows) in a table The basic structure of a DELETE query is as follows DELETE FROM table_name WHERE some_column=some_value;

Delete For example, say we want to delete the previously added student record, we might have the following DELETE FROM Students WHERE (Name = “Janet Smith”); Again – be careful, if you don’t set a WHERE condition you delete every record in the database Example

Joins Joins combine tuples (aka rows, records) from multiple tables Joins come in several forms  Inner Joins  Left Joins  Right Joins  Full Joins  Unions  Select Into/Into Select

Inner Joins Inner Joins return the specified columns at the intersection of two or more tables Image from

Inner Joins Inner Joins are the most basic (and probably most common) type of join The basic structure of an INNER JOIN query is as follows SELECT column_name(s) FROM table1 INNER JOIN table2 ON table1.column_name=table2.column_name;

Inner Joins For example, say in addition to the previously added student record, a separate REFERENCES table stores student details. We could retrieve Student_IDs included in both tables as follows: SELECT Students.Student_ID, Student.Name, References.Ref_Details FROM Students INNER JOIN References ON Students.Student_ID = References.Student_ID; Example

Left Joins Left Joins (sometimes called left outer joins) return all of the specified columns from the first table and their intersection (where it exists) with two or more tables Image from

Left Joins The big difference here is that columns from our first table that have no corresponding entry in the latter tables are still returned (with null signifying the missing entry) The basic structure of an LEFT JOIN query is as follows SELECT column_name(s) FROM table1 LEFT JOIN table2 ON table1.column_name=table2.column_name;

Left Joins For example, what if some students do not have references and we still want to see their Student_ID and Name? We could retrieve these records as follows: SELECT Students.Student_ID, Student.Name, References.Ref_Details FROM Students LEFTJOIN References ON Students.Student_ID = References.Student_ID; Example

Right Joins Right Joins (sometimes called right outer joins) return all of the specified columns from the latter tables and their intersection (where it exists) with the first table Image from

Right Joins Here, columns from our latter tables that have no corresponding entry in the first tables are still returned (with null signifying the missing entry in the first table) The basic structure of an RIGHT JOIN query is as follows SELECT column_name(s) FROM table1 RIGHT JOIN table2 ON table1.column_name=table2.column_name;

Right Joins For example, what if some references have been received before the corresponding student records have been created and we still want to see them? We could retrieve these records as follows: SELECT Students.Student_ID, Student.Name, References.Ref_Details FROM Students RIGHT JOIN References ON Students.Student_ID = References.Student_ID; Example ss=-1

Full Outer Joins Full Outer Joins return all of the specified columns from the first and latter tables, including entries in either table with no corresponding table in the other(s) Image from

Full Outer Joins Here, columns from our any table that have no corresponding entry in other tables are still returned (with null signifying the missing entry) The basic structure of an FULL OUTER JOIN query is as follows SELECT column_name(s) FROM table1 FULL OUTER JOIN table2 ON table1.column_name=table2.column_name;

Full Outer Joins For example, what if we want to see the full set of records to determine which student records we have not yet created and which references are still outstanding? We could retrieve these records as follows: SELECT Students.Student_ID, Student.Name, References.Ref_Details FROM Students FULL OUTER JOIN References ON Students.Student_ID = References.Student_ID;

Unions Unions are a bit different, they tend to be used for retrieving comprehensive sets of similar records Unions combine two or more SELECT queries, provided the following conditions are met  Each SELECT query must have the same number of columns  Each merged column must share data types  Columns in each SELECT query must be in the same order

Unions The basic structure of a UNION query is as follows SELECT column_name(s) FROM table1 UNION SELECT column_name(s) FROM table2; Note that this will automatically return only distinct records, though we can used UNION ALL if we want to include duplicates

Unions For example, what if our students table only stores enrolled students, whereas another Provisional_Students table stores students in provisional places awaiting confirmation? We could retrieve these records as follows: SELECT Name FROM Students UNION SELECT Name FROM Provisional_Students ORDER BY Name; Example

Exercise Consider the following problems related to the Customers database, what queries best solve them? 1. We want to retrieve all customer addresses in Mexico? 2. We want to add a new Customer called 'Juan Garcia Ramos', with contact name 'Juan Ramos', address of 'Tribulete 4356', in the city 'México D.F', with a post code of '05029', in the country of 'Mexico‘? 3. We want to update that customer’s contact name to ‘Anna Ramos’? 4. We want to delete the same customer?

Exercise 5. We want to retrieve all cities mentioned in customer records and supplier records using a full outer join 6. We want to retrieve all cities mentioned in customer records and supplier records using a union 7. We want to add all cities listed in the Suppliers table into the Customers table

Want to read more? Links and references  Tutorials on the PHP filestream