Structured Query Language Chris Nelson CS 157B Spring 2008

Overview Language Selections Joins Insertions Updates and Deletes Aggregate Functions Grouping SQL Training Resources

SQL Language SQL-1 Standardized by ANSI in 1986 Includes Data Definition Lang. (DDL) statements for Schema Modification Includes Data Manipulation Lang. (DML) statements for Data Manipulation SQL-2 Standardized in 1992 SQL-3 standardization still ongoing

Selections Selects  select from where  select is Relational algebra’s projection operation.  where is relational algebra’s selection operation.  Select does not maintain relational algebra’s uniqueness property. Hence the ‘ distinct ’ keyword.

Selection Criteria Numbers/Identities  Can be indicated with an equal sign Strings  Require single quotes  Can be wildcarded using % (multichar) _ (singlechar)  Depending on database implementation, may or may not have to worry about case. Oracle is case sensitive, SQL Server, Postgres, etc. are not

Joins Inner Join  Relational Algebra’s natural join. Joins primary keys on foreign keys. Matching elements will form a tuple in result list. Left/Right Joins  Allows additional null/non-matching columns to be included in result set. Side indicated in join will be the mandatory one. Outer Join  Union of left and right joins. Results will include inner join matches, as well as right and left mismatches.

Joins (Continued) The previous joins follow the format:  Select from tableA tableB on ie: select * from employee left join department on (employee.dept = dept.id); Also, manual joins can be used  Select from tableA, tableB where ie: select * from employee, department where employee.dept = dept.id; // note, not necessarily same result as above

Table Aliasing Used in manual joins. Useful for joining multiple tables, saving typing in where criteria. Follows format:  select.property from where.property = foo;  select p.name, r.balanceOwed from person p, record r where r.patientId = p.id;

Insertions Inserts  insert into tableName (attributes+) values (properties+);  Properties must follow same order as attributes  Can also chain with a select statement to handle a sort of “copy” operation insert into tableName (select…)

Updates and Deletes Updates  Update set = where Deletes  Delete from where Leaving out where criteria will delete/Update everything in the table

Aggregate Functions Count – returns count of matching tuples Average – returns average of specified attribue Max – returns max of specified attribute Min – returns min of specified attribute Sum – returns sum of all rows in table attribute Follow format: select fn( ) from where

Grouping Element grouping  Group by - groups common elements into one tuple  having - used to indicate criteria for grouping Query Unions (select * from xyz) union (select * from abc)

Schema Creation, Deletion Primary Keywords  Create create table { attribName … } usual attribute types include char, varChar(#), int.  Drop Cascade – delete all connected tables (ie: matching key pairs) Restrict – Delete only empty tables These have already been covered in-depth in previous presentations.

Common Errors Using double-quotes instead of single for string literals Forgetting escape characters for words featuring apostrophes ie: ‘o’reilly’ will terminate the string after o should be ‘o\’reilly’

Training Slides are nice, but practice is better than anything. Most databases have sample data sets available/installed (ie: SQL Server’s ‘Northwind’ database. Download here: yID= A0-8DA2- EEBC53A68034&displaylang=en) yID= A0-8DA2- EEBC53A68034&displaylang=en SQL Zoo Has a number of tests and tutorials (they’re actually kind of fun)

References and Recommended Links Principles of Database Systems with Internet and Java Applications, by Greg Riccardi MySQL Intro resources/articles/mysql_intro.htmlhttp://dev.mysql.com/tech- resources/articles/mysql_intro.html W3C School SQL intro