Chapter 12: Introduction to SQL
12.3 SQL Data Types
When working with SQL, it is important to understand the different data types that can be used to define each column in a table. Each data type determines what kind of data can be stored in the column. SQL supports various data types that can be categorized into three main categories: numeric types, date and time types, and string types.
Numeric data types are used to store numerical values, such as integers or decimals. Date and time types are used to store date and time information, such as dates, times of day, or both. String types, also known as character types, are used to store text-based data, such as names, addresses, or descriptions.
It is essential to choose the correct data type for each column based on the type of data that will be stored in it. Selecting an incorrect data type can lead to data loss, errors during data retrieval or insertion, and performance issues. Therefore, it is important to carefully consider the data types when designing a table in SQL.
12.3.1 Numeric Types
Numeric types include:
INTEGER: This is used for whole numbers.REAL: This is used for floating-point numbers.DECIMAL: This is used for precise fixed-point numbers.
12.3.2 Date and Time Types
Date and time types include:
DATE: This stores year, month, and day values.TIME: This stores hour, minute, and second values.DATETIME: This stores the date and time together in one column.
12.3.3 String Types
String types include:
CHAR: This is a string of a fixed length. If the string is less than the specified length, the remaining space is filled with blank spaces.VARCHAR: This is a string of a variable length. The maximum length is set by the user.TEXT: This is used for long text entries. The length of the string is variable and can be very large.
Let's look at an example where we create a table using these data types:
CREATE TABLE employees (
id INTEGER,
first_name VARCHAR(50),
last_name VARCHAR(50),
birth_date DATE,
hire_date DATE,
salary DECIMAL(7,2),
department VARCHAR(20)
);In this CREATE TABLE statement, we define a employees table with various columns each with their own data type.
12.3.4 SQL Constraints
SQL constraints are an essential feature of relational databases. They allow us to define strict rules that govern what data can be stored in a table, ensuring that the data is both accurate and reliable.
These constraints can be used to enforce a wide range of data rules, including limiting the values that can be entered into a column, ensuring that all records are unique, and making sure that data is entered in a specific format. By implementing SQL constraints, we can greatly improve the quality and consistency of the data in our database.
For example, we can use constraints to ensure that a customer's phone number is always in the correct format, or that a product's price is always greater than zero. Overall, SQL constraints are a powerful tool for maintaining data integrity and ensuring that our database is a reliable source of information.
Constraints can be column level or table level. Column level constraints apply to a column, and table level constraints apply to the whole table.
The following constraints are commonly used in SQL:
NOT NULL: Ensures that a column cannot have a NULL value.UNIQUE: Ensures that all values in a column are different.PRIMARY KEY: A combination of aNOT NULLandUNIQUE. Uniquely identifies each row in a table.FOREIGN KEY: Uniquely identifies a row/record in another table.CHECK: Ensures that all values in a column satisfy a specific condition.DEFAULT: Sets a default value for a column when none is specified.
Let's modify our previous CREATE TABLE statement to include some constraints:
CREATE TABLE employees (
id INTEGER PRIMARY KEY,
first_name VARCHAR(50) NOT NULL,
last_name VARCHAR(50) NOT NULL,
birth_date DATE,
hire_date DATE,
salary DECIMAL(7,2) CHECK(salary > 0),
department VARCHAR(20) DEFAULT 'UNKNOWN'
);12.3 SQL Data Types
When working with SQL, it is important to understand the different data types that can be used to define each column in a table. Each data type determines what kind of data can be stored in the column. SQL supports various data types that can be categorized into three main categories: numeric types, date and time types, and string types.
Numeric data types are used to store numerical values, such as integers or decimals. Date and time types are used to store date and time information, such as dates, times of day, or both. String types, also known as character types, are used to store text-based data, such as names, addresses, or descriptions.
It is essential to choose the correct data type for each column based on the type of data that will be stored in it. Selecting an incorrect data type can lead to data loss, errors during data retrieval or insertion, and performance issues. Therefore, it is important to carefully consider the data types when designing a table in SQL.
12.3.1 Numeric Types
Numeric types include:
INTEGER: This is used for whole numbers.REAL: This is used for floating-point numbers.DECIMAL: This is used for precise fixed-point numbers.
12.3.2 Date and Time Types
Date and time types include:
DATE: This stores year, month, and day values.TIME: This stores hour, minute, and second values.DATETIME: This stores the date and time together in one column.
12.3.3 String Types
String types include:
CHAR: This is a string of a fixed length. If the string is less than the specified length, the remaining space is filled with blank spaces.VARCHAR: This is a string of a variable length. The maximum length is set by the user.TEXT: This is used for long text entries. The length of the string is variable and can be very large.
Let's look at an example where we create a table using these data types:
CREATE TABLE employees (
id INTEGER,
first_name VARCHAR(50),
last_name VARCHAR(50),
birth_date DATE,
hire_date DATE,
salary DECIMAL(7,2),
department VARCHAR(20)
);In this CREATE TABLE statement, we define a employees table with various columns each with their own data type.
12.3.4 SQL Constraints
SQL constraints are an essential feature of relational databases. They allow us to define strict rules that govern what data can be stored in a table, ensuring that the data is both accurate and reliable.
These constraints can be used to enforce a wide range of data rules, including limiting the values that can be entered into a column, ensuring that all records are unique, and making sure that data is entered in a specific format. By implementing SQL constraints, we can greatly improve the quality and consistency of the data in our database.
For example, we can use constraints to ensure that a customer's phone number is always in the correct format, or that a product's price is always greater than zero. Overall, SQL constraints are a powerful tool for maintaining data integrity and ensuring that our database is a reliable source of information.
Constraints can be column level or table level. Column level constraints apply to a column, and table level constraints apply to the whole table.
The following constraints are commonly used in SQL:
NOT NULL: Ensures that a column cannot have a NULL value.UNIQUE: Ensures that all values in a column are different.PRIMARY KEY: A combination of aNOT NULLandUNIQUE. Uniquely identifies each row in a table.FOREIGN KEY: Uniquely identifies a row/record in another table.CHECK: Ensures that all values in a column satisfy a specific condition.DEFAULT: Sets a default value for a column when none is specified.
Let's modify our previous CREATE TABLE statement to include some constraints:
CREATE TABLE employees (
id INTEGER PRIMARY KEY,
first_name VARCHAR(50) NOT NULL,
last_name VARCHAR(50) NOT NULL,
birth_date DATE,
hire_date DATE,
salary DECIMAL(7,2) CHECK(salary > 0),
department VARCHAR(20) DEFAULT 'UNKNOWN'
);12.3 SQL Data Types
When working with SQL, it is important to understand the different data types that can be used to define each column in a table. Each data type determines what kind of data can be stored in the column. SQL supports various data types that can be categorized into three main categories: numeric types, date and time types, and string types.
Numeric data types are used to store numerical values, such as integers or decimals. Date and time types are used to store date and time information, such as dates, times of day, or both. String types, also known as character types, are used to store text-based data, such as names, addresses, or descriptions.
It is essential to choose the correct data type for each column based on the type of data that will be stored in it. Selecting an incorrect data type can lead to data loss, errors during data retrieval or insertion, and performance issues. Therefore, it is important to carefully consider the data types when designing a table in SQL.
12.3.1 Numeric Types
Numeric types include:
INTEGER: This is used for whole numbers.REAL: This is used for floating-point numbers.DECIMAL: This is used for precise fixed-point numbers.
12.3.2 Date and Time Types
Date and time types include:
DATE: This stores year, month, and day values.TIME: This stores hour, minute, and second values.DATETIME: This stores the date and time together in one column.
12.3.3 String Types
String types include:
CHAR: This is a string of a fixed length. If the string is less than the specified length, the remaining space is filled with blank spaces.VARCHAR: This is a string of a variable length. The maximum length is set by the user.TEXT: This is used for long text entries. The length of the string is variable and can be very large.
Let's look at an example where we create a table using these data types:
CREATE TABLE employees (
id INTEGER,
first_name VARCHAR(50),
last_name VARCHAR(50),
birth_date DATE,
hire_date DATE,
salary DECIMAL(7,2),
department VARCHAR(20)
);In this CREATE TABLE statement, we define a employees table with various columns each with their own data type.
12.3.4 SQL Constraints
SQL constraints are an essential feature of relational databases. They allow us to define strict rules that govern what data can be stored in a table, ensuring that the data is both accurate and reliable.
These constraints can be used to enforce a wide range of data rules, including limiting the values that can be entered into a column, ensuring that all records are unique, and making sure that data is entered in a specific format. By implementing SQL constraints, we can greatly improve the quality and consistency of the data in our database.
For example, we can use constraints to ensure that a customer's phone number is always in the correct format, or that a product's price is always greater than zero. Overall, SQL constraints are a powerful tool for maintaining data integrity and ensuring that our database is a reliable source of information.
Constraints can be column level or table level. Column level constraints apply to a column, and table level constraints apply to the whole table.
The following constraints are commonly used in SQL:
NOT NULL: Ensures that a column cannot have a NULL value.UNIQUE: Ensures that all values in a column are different.PRIMARY KEY: A combination of aNOT NULLandUNIQUE. Uniquely identifies each row in a table.FOREIGN KEY: Uniquely identifies a row/record in another table.CHECK: Ensures that all values in a column satisfy a specific condition.DEFAULT: Sets a default value for a column when none is specified.
Let's modify our previous CREATE TABLE statement to include some constraints:
CREATE TABLE employees (
id INTEGER PRIMARY KEY,
first_name VARCHAR(50) NOT NULL,
last_name VARCHAR(50) NOT NULL,
birth_date DATE,
hire_date DATE,
salary DECIMAL(7,2) CHECK(salary > 0),
department VARCHAR(20) DEFAULT 'UNKNOWN'
);12.3 SQL Data Types
When working with SQL, it is important to understand the different data types that can be used to define each column in a table. Each data type determines what kind of data can be stored in the column. SQL supports various data types that can be categorized into three main categories: numeric types, date and time types, and string types.
Numeric data types are used to store numerical values, such as integers or decimals. Date and time types are used to store date and time information, such as dates, times of day, or both. String types, also known as character types, are used to store text-based data, such as names, addresses, or descriptions.
It is essential to choose the correct data type for each column based on the type of data that will be stored in it. Selecting an incorrect data type can lead to data loss, errors during data retrieval or insertion, and performance issues. Therefore, it is important to carefully consider the data types when designing a table in SQL.
12.3.1 Numeric Types
Numeric types include:
INTEGER: This is used for whole numbers.REAL: This is used for floating-point numbers.DECIMAL: This is used for precise fixed-point numbers.
12.3.2 Date and Time Types
Date and time types include:
DATE: This stores year, month, and day values.TIME: This stores hour, minute, and second values.DATETIME: This stores the date and time together in one column.
12.3.3 String Types
String types include:
CHAR: This is a string of a fixed length. If the string is less than the specified length, the remaining space is filled with blank spaces.VARCHAR: This is a string of a variable length. The maximum length is set by the user.TEXT: This is used for long text entries. The length of the string is variable and can be very large.
Let's look at an example where we create a table using these data types:
CREATE TABLE employees (
id INTEGER,
first_name VARCHAR(50),
last_name VARCHAR(50),
birth_date DATE,
hire_date DATE,
salary DECIMAL(7,2),
department VARCHAR(20)
);In this CREATE TABLE statement, we define a employees table with various columns each with their own data type.
12.3.4 SQL Constraints
SQL constraints are an essential feature of relational databases. They allow us to define strict rules that govern what data can be stored in a table, ensuring that the data is both accurate and reliable.
These constraints can be used to enforce a wide range of data rules, including limiting the values that can be entered into a column, ensuring that all records are unique, and making sure that data is entered in a specific format. By implementing SQL constraints, we can greatly improve the quality and consistency of the data in our database.
For example, we can use constraints to ensure that a customer's phone number is always in the correct format, or that a product's price is always greater than zero. Overall, SQL constraints are a powerful tool for maintaining data integrity and ensuring that our database is a reliable source of information.
Constraints can be column level or table level. Column level constraints apply to a column, and table level constraints apply to the whole table.
The following constraints are commonly used in SQL:
NOT NULL: Ensures that a column cannot have a NULL value.UNIQUE: Ensures that all values in a column are different.PRIMARY KEY: A combination of aNOT NULLandUNIQUE. Uniquely identifies each row in a table.FOREIGN KEY: Uniquely identifies a row/record in another table.CHECK: Ensures that all values in a column satisfy a specific condition.DEFAULT: Sets a default value for a column when none is specified.
Let's modify our previous CREATE TABLE statement to include some constraints:
CREATE TABLE employees (
id INTEGER PRIMARY KEY,
first_name VARCHAR(50) NOT NULL,
last_name VARCHAR(50) NOT NULL,
birth_date DATE,
hire_date DATE,
salary DECIMAL(7,2) CHECK(salary > 0),
department VARCHAR(20) DEFAULT 'UNKNOWN'
);