MAX() Aggregate Function
MAX() aggregate function takes the name of a column as an argument and returns the largest value in a column. The given query will return the largest value from the
SELECT MAX(amount) FROM transactions;
SELECT * statement returns all columns from the provided table in the result set. The given query will fetch all columns and records (rows) from the
SELECT * FROM movies;
ORDER BY Clause
ORDER BY clause can be used to sort the result set by a particular column either alphabetically or numerically. It can be ordered in two ways:
DESCis a keyword used to sort the results in descending order.
ASCis a keyword used to sort the results in ascending order (default).
SELECT * FROM contacts ORDER BY birth_date DESC;
COUNT() Aggregate Function
COUNT() aggregate function returns the total number of rows that match the specified criteria. For instance, to find the total number of employees who have less than 5 years of experience, the given query can be used.
Note: A column name of the table can also be used instead of
COUNT(*), this variation
COUNT(column) will not count
NULL values in that column.
SELECT COUNT(*) FROM employees WHERE experience < 5;
Unique values of a column can be selected using a
DISTINCT query. For a table
contact_details having five rows in which the
city column contains Chicago, Madison, Boston, Madison, and Denver, the given query would return:
SELECT DISTINCT city FROM contact_details;
LIMIT clause is used to narrow, or limit, a result set to the specified number of rows. The given query will limit the result set to 5 rows.
SELECT * FROM movies LIMIT 5;
GROUP BY Clause
GROUP BY clause will group records in a result set by identical values in one or more columns. It is often used in combination with aggregate functions to query information of similar records. The
GROUP BY clause can come after
WHERE but must come before any
ORDER BY or
The given query will count the number of movies per rating.
SELECT rating, COUNT(*) FROM movies GROUP BY rating;
MIN() Aggregate Function
MIN() aggregate function returns the smallest value in a column. For instance, to find the smallest value of the
amount column from the table named
transactions, the given query can be used.
SELECT MIN(amount) FROM transactions;
CASE statement in SQL
CASE statement enables control flow in SQL. It allows for one or more conditions (
THEN result) and an optional default case (
ELSE). The query above will provide each rating a value for the specified ranges within the result set.
SELECT name, CASE WHEN rating > 8 THEN "Excellent" WHEN rating > 5 THEN "Good" WHEN rating > 3 THEN "Okay" ELSE "Bad" END FROM movies;
HAVING clause is used to further filter the result set groups provided by the
GROUP BY clause.
HAVING is often used with aggregate functions to filter the result set groups based on an aggregate property. The given query will select only the records (rows) from only years where more than 5 movies were released per year.
SELECT year, COUNT(*) FROM movies GROUP BY year HAVING COUNT(*) > 5;
WHERE clause is used to filter records (rows) that match a certain condition. The given query will select all records where the
SELECT title FROM library WHERE pub_year = 2017;
ROUND() function will round a number value to a specified number of places. It takes two arguments: a number, and a number of decimal places. It can be combined with other aggregate functions, as shown in the given query. This query will calculate the average rating of movies from 2015, rounding to 2 decimal places.
SELECT year, ROUND(AVG(rating), 2) FROM movies WHERE year = 2015;
An outer join will combine rows from different tables even if the join condition is not met. In a
LEFT JOIN, every row in the left table is returned in the result set, and if the join condition is not met, then
NULL values are used to fill in the columns from the right table.
SELECT column_name(s) FROM table1 LEFT JOIN table2 ON table1.column_name = table2.column_name;
JOIN clause allows for the return of results from more than one table by joining them together with other results based on common column values specified using an
INNER JOIN is the default
JOIN and it will only return results matching the condition specified by
SELECT * FROM books JOIN authors ON books.author_id = authors.id;
A primary key column in a SQL table is used to uniquely identify each record in that table. A primary key cannot be
NULL. In the example,
customer_id is the primary key. The same value cannot re-occur in a primary key column. Primary keys are often used in
UPDATE statement is used to edit records (rows) in a table. It includes a
SET clause that indicates the column to edit and a
WHERE clause for specifying the record(s).
UPDATE table_name SET column1 = value1, column2 = value2 WHERE some_column = some_value;
ALTER TABLE Statement
ALTER TABLE statement is used to modify the columns of an existing table. When combined with the
ADD COLUMN clause, it is used to add a new column.
ALTER TABLE table_name ADD column_name datatype;
Column constraints are the rules applied to the values of individual columns:
PRIMARY KEYconstraint can be used to uniquely identify the row.
UNIQUEcolumns have a different value for every row.
NOT NULLcolumns must have a value.
DEFAULTassigns a default value for the column when no value is specified.
There can be only one
PRIMARY KEY column per table and multiple
CREATE TABLE student ( id INTEGER PRIMARY KEY, name TEXT UNIQUE, grade INTEGER NOT NULL, age INTEGER DEFAULT 10 );
Data Types As Constraints
Columns of a PostgreSQL database table must have a data type, which constrains the type of information that can be entered into that column. This is important in order to ensure data integrity and consistency over time. Some common PostgreSQL types are
boolean. Data types are defined in a
CREATE TABLE statement by indicating the data type after each column name.
CREATE TABLE tablename ( myNum integer, myString varchar(50) );
NOT NULL constraints can be used to ensure that particular columns of a database table do not contain missing data. This is important for ensuring database integrity and consistency over time.
NOT NULL constraints can be enforced within a
CREATE TABLE statement using
CREATE TABLE table_name ( column_1 integer NOT NULL, column_2 text NOT NULL, column_3 numeric );
UNIQUE constraints can be used to ensure that elements of a particular column (or group of columns) are unique (i.e., no two rows have the same value or combination of values). This is important for ensuring database integrity and consistency over time.
UNIQUE constraints can be enforced within a
CREATE TABLE statement using the
CREATE TABLE table_name ( column_1 integer UNIQUE, column_2 text UNIQUE, column_3 numeric, column_4 text, UNIQUE(column_3, column_4) );
The primary key of a database table is a column or group of columns that can be used to uniquely identify every row of the table. For example, a table of students might have a primary key named
student_id, which contains unique ID numbers for each student.
One-to-One Database Relationships
In a relational database, two tables have a one-to-one relationship if each row in one table links to exactly one row in the other table, and vice versa. For example, a table of
employees and a table of
employee_contact_info might have a one-to-one relationship if every employee listed in the
employees table has contact information listed in the
employee_contact_info table and vice versa.