Structured Query Language and its components

SQL SQL stands for Structured Query Language. There is a standard SQL called the American National Standards Institute’s 2003 Standard SQL (ANSI:2003 SQL) Most database vendors cover much of the standard, but do not adhere to it completely. –Note, when you start using Oracle SQL, you will see that many of the functions are different.

Database Vendors While there is a standard, there are many providers of database systems. Some of the providers you may find: –Oracle –Microsoft SQL Server –Microsoft Access (very limited) –MySQL (open source – popular for projects) –PostGres (open source)

Some definitions Before explaining SQL, let’s look at some definitions: –Metadata: This is data about data, i.e. table definitions, column definitions, etc. –Data: This is the value that is held in the database, that must follow the rules of the metadata. –Transaction: A transaction is a unit of work, passed to the database for processing. –Session: A process that connects to the database, relating an individual user to a specific database (or schema) allowing the user to interact with the database, ending when the user disconnects from the database.

SQL – What’s in it? SQL is made up of different categories of commands: –Data Definition Language –Data Manipulation Language –Transaction control statements –Session or data control statements

Data Definition Language (DDL) This consists of commands that enable the database administrator, in association with the application developer, to manipulate the infrastructure of the database. This infrastructure is known as the conceptual schema. It enables definition of the metadata. The commands used to do this are: –CREATE –DROP –ALTER –TRUNCATE

Data Manipulation Language (DML) This consists of commands that enable the application developer to manipulate the data in the tables. The commands used are: –SELECT –INSERT –UPDATE –DELETE –MERGE

Transaction Control Statements These statements allow the application developer to group DML statements, in order to conduct a transaction. A transaction is a unit of work, passed to the database for processing. A transaction will often require: –Selection from one or more tables or views –Insertion to one or more tables or views –Update to one or more tables or views –Deletion from one or more tables. Examples: –Place an order for several items –Register as a student –Pay a phone bill.

Transaction Control Statements Most of the statements used during a transaction are DML statements (some DDL statements may be used). There are also transaction safeguard statements. These are: –Commit –Rollback

Data Control Language (DCL) These consist of statements that allow the database (schema) owner to control either his / her own sessions, or sessions of other users trying to access his / her data. The statements are: –GRANT –REVOKE –And various SET commands (Session control)

So far we have met… SELECT INSERT CREATE DROP Let’s look back at them.

SELECT This statement is very versatile and is the single retrieval mechanism. Its basic components are –SELECT –Field-list –FROM –Table-list We have tried these in the labs, and will continue to do so.

SELECT field-list The field-list in a SELECT statement can be: –A wildcard character * to denote all eligible fields. –A column name that is unique to one of the tables in the table list. –A table-name.column-name to specify a column from a specific table from the table list. –A database.table-name.column-name to specify a column in a specific table in a specific database Please note: Oracle calls the domain in which the tables reside a schema. SQL Server calls this domain a database. –A derived field

Derived fields in the SELECT field- list Fields can be derived by: –Performing calculations on column-fields from the table-list. E.g. unitprice * quantity as linecost –Using functions on the column-fields from the table list. E.g. day(orderdate) –Using database provided functions. E.g. SQL Server: getdate() gets the current date from the system, not from any table. Oracle: Select sysdate from dual; (Dual is a working storage area for use in sessions connected to the database).

Table-list Initially, the queries we do will be on single tables, but as we get more fluent with SQL, we will start to do multi-table selects. The tables must belong to the same database / schema. Sometimes the database/schema name is required to qualify the table name. –See later.

Additions to Select If you think of a table (e.g. Dog) as a 2D grid, the column- list manipulates the columns: DogIdNameWeightAgeDietExerciseBreed_Id 1Goldie403Standard Glab 2Mutt462Standard Glab 3Spot444Standard Pood 4Sooty555Standard Blab 5Beauty503Specialised Grtv 6Jack674Specialised GDan 7Pal555Standard Blab I’ve shortened the column names here to fit it in the slide.

Select name, age from dog This query picks out specific columns from the table. DogIdNameWeightAgeDietExercise Breed_I d 1Goldie403Standard Glab 2Mutt462Standard Glab 3Spot444Standard Pood 4Sooty555Standard Blab 5Beauty503 Specialise d Grtv 6Jack674 Specialise d GDan 7Pal555Standard Blab

To pick out rows: This is called projection and is done using the WHERE clause. This clause goes after the basic select: SELECT column-list FROM table-list WHERE condition The condition usually relates to a value in one or more of the columns from the column list.

Conditions The conditions can include: >,, =, !=, NOT, between, IS NULL, IS LIKE. The IS NULL returns a true if the value in the column is null, and a false otherwise. You will NEVER get anything if you use the condition WHERE column = NULL –Null means undefined. You cannot equate to undefined!

LIKE LIKE allows us to match patterns in strings. Wildcard characters can be used to represent –A character from a string _ –A variable length substring from a string %.

Manipulating ROWS The WHERE clause allows to choose from specific ROWS in our query: SELECT dogname, dogage FROM dog WHERE dogage BETWEEN 2 and 4 Look back at the exercises we did last week.

Formatting output We looked at date formatting last week in the lab. What other formatting may we want? –String formatting Concatenation: –SQL Server + –Oracle || Renaming –AS clause SELECT corder.unitprice as Price or –SELECT corder.unitprice as ‘Unit Price’ Numeric formatting (use cast, convert (SQL Server) and to_char(Oracle).

Ordering your data To sort the output by a particular column, add the suffix ORDER BY column-name E.g. SELECT dogname, dogage FROM dog ORDER BY dogage DognameDogAge Mutt2 Goldie3 Beauty3 Jack4 Spot4 Sooty5 Pal5

Ordering your data You can reverse the order: SELECT dogname, dogage FROM dog ORDER BY dogage DESC DognameDogAge Sooty5 Pal5 Jack4 Spot4 Goldie3 Beauty3 Mutt2

Ordering your data You can also order it by two columns: SELECT dogname, dogage FROM dog ORDER BY dogage, dogname DESC DognameDogAge Mutt2 Goldie3 Beauty3 Spot4 Jack4 Sooty5 Pal5