SQL UNIQUE Contraints
The UNIQUE Contraints identifies a record in a database table uniquely. Unlike PRIMARY KEY contraints, you can have many UNIQUE contraint in a table.
UNIQUE Contraints Syntax - MySQL
SQL Union Operator
The UNION Operator is used to combine the result set of two or more SELECT Statement. This operator is useful if you have two table that share the same column name and data type and you want to combine it in a single Query.
Take note that both table must have the same column name and data type.
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SQL DEFAULT Constraint
The DEFAULT Constraint is used to insert the default values into a column during design time.
The default values is used in new record if no value is specified during INSERT Statement.
The default
SQL DEFAULT Constraint Syntax
SQL COUNT() Function
COUNT is a simple aggregate function provided by SQL. As it is clear from its name that it simply counts the number of records in the table and returns the total count.
There are three (3) different usage for the Count function.
- SQL COUNT(column_name)
- COUNT(*)
- COUNT(DISTINCT column_name)
Consider the following table for this exercise
Employess
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SQL Data Types
The data type determines the kind of values that users can store in the field. Like every programming languages, the SQL also has specific data type to store data. Following are some data type and there descriptions in which SQL allows us to store data.
Microsoft Access Data Types
Source: Microsoft Access Help
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SQL AUTO INCREMENT Field
The AUTO INCREMENT generates a new number that increment by 1 from the previous number. This will allow a unique number to be associated in a row when it is inserted into a table.
You can set the AUTO INCREMENT during creation of the table or you can also use ALTER TABLE Statement if you have already created the table.
The following example will create a table named "Users" and add a Primary Key with Auto Increment in ID field.
Syntax for SQL Server