Introduction to SQL

Editor's Notes

• #6: SQL is Structured Query Language, which is a computer language for storing, manipulating and retrieving data stored in a relational database. SQL is the standard language for Relational Database System. All the Relational Database Management Systems (RDMS) like MySQL, MS Access, Oracle, Sybase, Informix, Postgres and SQL Server use SQL as their standard database language. Also, they are using different dialects, such as − MS SQL Server using T-SQL, Oracle using PL/SQL, MS Access version of SQL is called JET SQL (native format) etc.
• #7: What Can SQL do? SQL can execute queries against a database SQL can retrieve data from a database SQL can insert records in a database SQL can update records in a database SQL can delete records from a database
• #8: What Can SQL do? SQL can create new databases SQL can create new tables in a database SQL can create stored procedures in a database SQL can create views in a database SQL can set permissions on tables, procedures, and views
• #9: SQL is a Standard - BUT.... Although SQL is an American National Standards Institute (ANSI) /International Organization for Standardization (ISO) standard, there are different versions of the SQL language. However, to be compliant with the ANSI standard, they all support at least the major commands (such as SELECT, UPDATE, DELETE, INSERT, WHERE) in a similar manner.
• #10: SQL Process When you are executing an SQL command for any RDBMS, the system determines the best way to carry out your request and SQL engine figures out how to interpret the task. There are various components included in this process. These components are − Query Dispatcher Optimization Engines Classic Query Engine SQL Query Engine, etc.
• #12: SQL Commands The standard SQL commands to interact with relational databases are CREATE, SELECT, INSERT, UPDATE, DELETE and DROP. These commands can be classified into the following groups based on their nature −
• #16: RDBMS stands for Relational Database Management System. RDBMS is the basis for SQL, and for all modern database systems like MS SQL Server, IBM DB2, Oracle, MySQL, and Microsoft Access. A Relational database management system (RDBMS) is a database management system (DBMS) that is based on the relational model as introduced by E. F. Codd.
• #17: What is a table? The data in an RDBMS is stored in database objects which are called as tables. This table is basically a collection of related data entries and it consists of numerous columns and rows. Remember, a table is the most common and simplest form of data storage in a relational database. The following program is an example of a CUSTOMERS table −
• #18: What is a table? The data in an RDBMS is stored in database objects which are called as tables. This table is basically a collection of related data entries and it consists of numerous columns and rows. Remember, a table is the most common and simplest form of data storage in a relational database. The following program is an example of a CUSTOMERS table −
• #22: A NULL value in a table is a value in a field that appears to be blank, which means a field with a NULL value is a field with no value. It is very important to understand that a NULL value is different than a zero value or a field that contains spaces. A field with a NULL value is the one that has been left blank during a record creation.
• #23: SQL Constraints Constraints are the rules enforced on data columns on a table. These are used to limit the type of data that can go into a table. This ensures the accuracy and reliability of the data in the database. Constraints can either be column level or table level. Column level constraints are applied only to one column whereas, table level constraints are applied to the entire table.
• #24: Following are some of the most commonly used constraints available in SQL − NOT NULL Constraint − Ensures that a column cannot have a NULL value. DEFAULT Constraint − Provides a default value for a column when none is specified.
• #25: Following are some of the most commonly used constraints available in SQL − UNIQUE Constraint − Ensures that all the values in a column are different. PRIMARY Key − Uniquely identifies each row/record in a database table.
• #26: Following are some of the most commonly used constraints available in SQL − INDEX − Used to create and retrieve data from the database very quickly.
• #27: Following are some of the most commonly used constraints available in SQL − INDEX − Used to create and retrieve data from the database very quickly.
• #28: Data Integrity The following categories of data integrity exist with each RDBMS − Entity Integrity − There are no duplicate rows in a table. Domain Integrity − Enforces valid entries for a given column by restricting the type, the format, or the range of values.
• #29: Data Integrity The following categories of data integrity exist with each RDBMS − Referential integrity − Rows cannot be deleted, which are used by other records. User-Defined Integrity − Enforces some specific business rules that do not fall into entity, domain or referential integrity.
• #30: Database Normalization Database normalization is the process of efficiently organizing data in a database. There are two reasons of this normalization process − Eliminating redundant data, for example, storing the same data in more than one table. Ensuring data dependencies make sense. Both these reasons are worthy goals as they reduce the amount of space a database consumes and ensures that data is logically stored. Normalization consists of a series of guidelines that help guide you in creating a good database structure. It is your choice to take it further and go to the fourth normal form, fifth normal form and so on, but in general, the third normal form is more than enough. First Normal Form (1NF) Second Normal Form (2NF) Third Normal Form (3NF)
• #31: SQL Data Type is an attribute that specifies the type of data of any object. Each column, variable and expression has a related data type in SQL. You can use these data types while creating your tables. You can choose a data type for a table column based on your requirement.
• #32: SQL Data Type is an attribute that specifies the type of data of any object. Each column, variable and expression has a related data type in SQL. You can use these data types while creating your tables. You can choose a data type for a table column based on your requirement.
• #33: Note − Here, datetime has 3.33 milliseconds accuracy where as smalldatetime has 1 minute accuracy.
• #34: DATA TYPE & Description char Maximum length of 8,000 characters.( Fixed length non-Unicode characters) varchar Maximum of 8,000 characters.(Variable-length non-Unicode data). varchar(max) Maximum length of 2E + 31 characters, Variable-length non-Unicode data (SQL Server 2005 only). text Variable-length non-Unicode data with a maximum length of 2,147,483,647 characters. UNICODE nchar Maximum length of 4,000 characters.( Fixed length Unicode) nvarchar Maximum length of 4,000 characters.(Variable length Unicode) nvarchar(max) Maximum length of 2E + 31 characters (SQL Server 2005 only).( Variable length Unicode) ntext Maximum length of 1,073,741,823 characters. ( Variable length Unicode ) Binary Data Types binary Maximum length of 8,000 bytes(Fixed-length binary data ) varbinary Maximum length of 8,000 bytes.(Variable length binary data) varbinary(max) Maximum length of 2E + 31 bytes (SQL Server 2005 only). ( Variable length Binary data) image Maximum length of 2,147,483,647 bytes. ( Variable length Binary Data)
• #35: Misc Data Types sql_variant Stores values of various SQL Server-supported data types, except text, ntext, and timestamp. timestamp Stores a database-wide unique number that gets updated every time a row gets updated uniqueidentifier Stores a globally unique identifier (GUID) xml Stores XML data. You can store xml instances in a column or a variable (SQL Server 2005 only). cursor Reference to a cursor object table Stores a result set for later processing
• #36: An operator is a reserved word or a character used primarily in an SQL statement's WHERE clause to perform operation(s), such as comparisons and arithmetic operations. These Operators are used to specify conditions in an SQL statement and to serve as conjunctions for multiple conditions in a statement. Arithmetic operators Comparison operators Logical operators Operators used to negate conditions
• #37: Arithmetic operators Comparison operators Logical operators Operators used to negate conditions
• #38: Arithmetic operators + (Addition) Adds values on either side of the operator. a + b will give 30 - (Subtraction) Subtracts right hand operand from left hand operand. a - b will give -10 * (Multiplication) Multiplies values on either side of the operator. a * b will give 200 / (Division) Divides left hand operand by right hand operand. b / a will give 2 % (Modulus) Divides left hand operand by right hand operand and returns remainder. b % a will give 0
• #39: Comparison operators = Checks if the values of two operands are equal or not, if yes then condition becomes true. (a = b) is not true. != Checks if the values of two operands are equal or not, if values are not equal then condition becomes true. (a != b) is true. <>  Checks if the values of two operands are equal or not, if values are not equal then condition becomes true. (a <> b) is true. >  Checks if the value of left operand is greater than the value of right operand, if yes then condition becomes true. (a > b) is not true. <  Checks if the value of left operand is less than the value of right operand, if yes then condition becomes true. (a < b) is true. >= Checks if the value of left operand is greater than or equal to the value of right operand, if yes then condition becomes true. (a >= b) is not true. <= Checks if the value of left operand is less than or equal to the value of right operand, if yes then condition becomes true. (a <= b) is true. !< Checks if the value of left operand is not less than the value of right operand, if yes then condition becomes true. (a !< b) is false. !> Checks if the value of left operand is not greater than the value of right operand, if yes then condition becomes true. (a !> b) is true.
• #40: Logical operators 1 ALL The ALL operator is used to compare a value to all values in another value set. 2 AND The AND operator allows the existence of multiple conditions in an SQL statement's WHERE clause. 3 ANY The ANY operator is used to compare a value to any applicable value in the list as per the condition. 4 BETWEEN The BETWEEN operator is used to search for values that are within a set of values, given the minimum value and the maximum value. 5 EXISTS The EXISTS operator is used to search for the presence of a row in a specified table that meets a certain criterion. 6 IN The IN operator is used to compare a value to a list of literal values that have been specified. 7 LIKE The LIKE operator is used to compare a value to similar values using wildcard operators. 8 NOT The NOT operator reverses the meaning of the logical operator with which it is used. Eg: NOT EXISTS, NOT BETWEEN, NOT IN, etc. This is a negate operator. 9 OR The OR operator is used to combine multiple conditions in an SQL statement's WHERE clause. 10 IS NULL The NULL operator is used to compare a value with a NULL value. 11 UNIQUE The UNIQUE operator searches every row of a specified table for uniqueness (no duplicates).