NULL values represent missing unknown data. By default, a table column can hold NULL values. This chapter will explain the IS NULL and IS NOT NULL operators. SQL NULL Values If a column in a table is optional, we can insert a new record or update an existing record without adding a value to this column. This means that the field will be saved with a NULL value. NULL values are treated differently from other values. NULL is used…
As long as your data contains only the date portion, your queries will work as expected. However, if a time portion is involved, it gets complicated. Before talking about the complications of querying for dates, we will look at the most important built-in functions for working with dates. MySQL Date Functions The following table lists the most important built-in date functions in MySQL: Function Description NOW() Returns the current date and time CURDATE() Returns the current date CURTIME() Returns…
A view is a virtual table. This chapter shows how to create, update, and delete a view. SQL CREATE VIEW Statement In SQL, a view is a virtual table based on the result-set of an SQL statement. A view contains rows and columns, just like a real table. The fields in a view are fields from one or more real tables in the database. You can add SQL functions, WHERE, and JOIN statements to a view and present the…
Auto-increment allows a unique number to be generated when a new record is inserted into a table. AUTO INCREMENT a Field Very often we would like the value of the primary key field to be created automatically every time a new record is inserted. We would like to create an auto-increment field in a table. Syntax for MySQL The following SQL statement defines the “ID” column to be an auto-increment primary key field in the “Persons” table: CREATE TABLE…
The ALTER TABLE statement is used to add, delete, or modify columns in an existing table. SQL ALTER TABLE Syntax To add a column in a table, use the following syntax: ALTER TABLE table_name ADD column_name datatype To delete a column in a table, use the following syntax (notice that some database systems don’t allow deleting a column): ALTER TABLE table_name DROP COLUMN column_name To change the data type of a column in a table, use the following syntax:…
Indexes, tables, and databases can easily be deleted/removed with the DROP statement. The DROP INDEX Statement The DROP INDEX statement is used to delete an index in a table. DROP INDEX Syntax for MS Access: DROP INDEX index_name ON table_name DROP INDEX Syntax for MS SQL Server: DROP INDEX table_name.index_name DROP INDEX Syntax for DB2/Oracle: DROP INDEX index_name DROP INDEX Syntax for MySQL: ALTER TABLE table_name DROP INDEX index_name The DROP TABLE Statement The DROP TABLE statement is…
The CREATE INDEX statement is used to create indexes in tables. Indexes allow the database application to find data fast; without reading the whole table. Indexes An index can be created in a table to find data more quickly and efficiently. The users cannot see the indexes, they are just used to speed up searches/queries. Note: Updating a table with indexes takes more time than updating a table without (because the indexes also need an update). So you should only…
The DEFAULT constraint is used to insert a default value into a column. The default value will be added to all new records, if no other value is specified. SQL DEFAULT Constraint on CREATE TABLE The following SQL creates a DEFAULT constraint on the “City” column when the “Persons” table is created: My SQL / SQL Server / Oracle / MS Access: CREATE TABLE Persons ( P_Id int NOT NULL, LastName varchar(255) NOT NULL, FirstName varchar(255), Address varchar(255), City…
SQL CHECK Constraint The CHECK constraint is used to limit the value range that can be placed in a column. If you define a CHECK constraint on a single column it allows only certain values for this column. If you define a CHECK constraint on a table it can limit the values in certain columns based on values in other columns in the row. SQL CHECK Constraint on CREATE TABLE The following SQL creates a CHECK constraint on the…
SQL FOREIGN KEY Constraint A FOREIGN KEY in one table points to a PRIMARY KEY in another table. Let’s illustrate the foreign key with an example. Look at the following two tables: The “Persons” table: P_Id LastName FirstName Address City 1 Hansen Ola Timoteivn 10 Sandnes 2 Svendson Tove Borgvn 23 Sandnes 3 Pettersen Kari Storgt 20 Stavanger The “Orders” table: O_Id OrderNo P_Id 1 77895 3 2 44678 3 3 22456 2 4 24562 1 Note that the…
