To get a data job, you are going to need to learn SQL. These common SQL commands and operators are a great reference.
Below is a comprehensive list of SQL commands, organized by the top-level of each (e.g. SELECT TOP is within the SELECT category).
If you’re on a journey to learn SQL and you’ve been frustrated by the lack of structure or the dull curriculum, then you may like Dataquest’s interactive SQL courses. Try it free.
Here are a few SQL courses:
- Introduction to SQL and Databases
- Filtering and Sorting Data in SQL
- Summarizing Data in SQL
- Combining Tables in SQL
- SQL Subqueries
List of SQL Commands
SELECT
SELECT is probably the most commonly-used SQL statement. You'll use it pretty much every time you query data with SQL. It allows you to define what data you want your query to return.
For example, in the code below, we’re selecting a column called name from a table called customers.
SELECT nameFROM customers;SELECT *
SELECT used with an asterisk (*) will return all of the columns in the table we're querying.
SELECT * FROM customers;SELECT DISTINCT
SELECT DISTINCT only returns data that is distinct — in other words, if there are duplicate records, it will return only one copy of each.
The code below would return only rows with a unique name from the customers table.
SELECT DISTINCT nameFROM customers;SELECT INTO
SELECT INTO copies the specified data from one table into another.
SELECT * INTO customersFROM customers_backup;SELECT TOP
SELECT TOP only returns the top x number or percent from a table.
The code below would return the top 50 results from the customers table:
SELECT TOP 50 * FROM customers;The code below would return the top 50 percent of the customers table:
SELECT TOP 50 PERCENT * FROM customers;AS
AS renames a column or table with an alias that we can choose. For example, in the code below, we’re renaming the name column as first_name:
SELECT name AS first_nameFROM customers;FROM
FROM specifies the table we're pulling our data from:
SELECT nameFROM customers;WHERE
WHERE filters your query to only return results that match a set condition. We can use this together with conditional operators like =, >, <, >=, <=, etc.
SELECT nameFROM customersWHERE name = ‘Bob’;AND
AND combines two or more conditions in a single query. All of the conditions must be met for the result to be returned.
SELECT nameFROM customersWHERE name = ‘Bob’ AND age = 55;OR
OR combines two or more conditions in a single query. Only one of the conditions must be met for a result to be returned.
SELECT nameFROM customersWHERE name = ‘Bob’ OR age = 55;BETWEEN
BETWEEN filters your query to return only results that fit a specified range.
SELECT nameFROM customersWHERE age BETWEEN 45 AND 55;LIKE
LIKE searches for a specified pattern in a column. In the example code below, any row with a name that included the characters Bob would be returned.
SELECT nameFROM customersWHERE name LIKE ‘%Bob%’;Other operators for LIKE:
%x— will select all values that begin with x%x%— will select all values that include xx%— will select all values that end with xx%y— will select all values that begin with x and end with y_x%— will select all values have x as the second characterx_%— will select all values that begin with x and are at least two characters long. You can add additional _ characters to extend the length requirement, i.e.x___%
IN
IN allows us to specify multiple values we want to select for when using the WHERE command.
SELECT nameFROM customersWHERE name IN (‘Bob’, ‘Fred’, ‘Harry’);IS NULL
IS NULL will return only rows with a NULL value.
SELECT nameFROM customersWHERE name IS NULL;IS NOT NULL
IS NOT NULL does the opposite — it will return only rows without a NULL value.
SELECT nameFROM customersWHERE name IS NOT NULL;CREATE
CREATE can be used to set up a database, table, index or view.
CREATE DATABASE
CREATE DATABASE creates a new database, assuming the user running the command has the correct admin rights.
CREATE DATABASE dataquestDB;CREATE TABLE
CREATE TABLE creates a new table inside a database. The terms int and varchar(255) in this example specify the datatypes of the columns we're creating.
CREATE TABLE customers ( customer_id int, name varchar(255), age int);CREATE INDEX
CREATE INDEX generates an index for a table. Indexes are used to retrieve data from a database faster.
CREATE INDEX idx_nameON customers (name);CREATE VIEW
CREATE VIEW creates a virtual table based on the result set of an SQL statement. A view is like a regular table (and can be queried like one), but it isnot saved as a permanent table in the database.
CREATE VIEW [Bob Customers] ASSELECT name, ageFROM customersWHERE name = ‘Bob’;DROP
DROP statements can be used to delete entire databases, tables or indexes.
It goes without saying that the DROP command should only be used where absolutely necessary.
DROP DATABASE
DROP DATABASE deletes the entire database including all of its tables, indexes etc as well as all the data within it.
Again, this is a command we want to be very, very careful about using!
DROP DATABASE dataquestDB;DROP TABLE
DROP TABLE deletes a table as well as the data within it.
DROP TABLE customers;DROP INDEX
DROP INDEX deletes an index within a database.
DROP INDEX idx_name;UPDATE
The UPDATE statement is used to update data in a table. For example, the code below would update the age of any customer named Bob in the customers table to 56.
UPDATE customersSET age = 56WHERE name = ‘Bob’;DELETE
DELETE can remove all rows from a table (using ), or can be used as part of a WHERE clause to delete rows that meet a specific condition.
DELETE FROM customersWHERE name = ‘Bob’;ALTER TABLE
ALTER TABLE allows you to add or remove columns from a table. In the code snippets below, we’ll add and then remove a column for surname. The text varchar(255) specifies the datatype of the column.
ALTER TABLE customersADD surname varchar(255);ALTER TABLE customersDROP COLUMN surname;AGGREGATE FUNCTIONS (COUNT/SUM/AVG/MIN/MAX)
An aggregate function performs a calculation on a set of values and returns a single result.
COUNT
COUNT returns the number of rows that match the specified criteria. In the code below, we’re using *, so the total row count for customers would be returned.
SELECT COUNT(*)FROM customers;SUM
SUM returns the total sum of a numeric column.
SELECT SUM(age)FROM customers;AVG
AVG returns the average value of a numeric column.
SELECT AVG(age)FROM customers;MIN
MIN returns the minimum value of a numeric column.
SELECT MIN(age)FROM customers;MAX
MAX returns the maximum value of a numeric column.
SELECT MAX(age)FROM customers;GROUP BY
The GROUP BY statement groups rows with the same values into summary rows. The statement is often used with aggregate functions. For example, the code below will display the average age for each name that appears in our customers table.
SELECT name, AVG(age)FROM customersGROUP BY name;HAVING
HAVING performs the same action as the WHERE clause. The difference is that HAVING is used for aggregate functions, whereas WHERE doesn’t work with them.
The below example would return the number of rows for each name, but only for names with more than 2 records.
SELECT COUNT(customer_id), nameFROM customersGROUP BY nameHAVING COUNT(customer_id) > 2;ORDER BY
ORDER BY sets the order of the returned results. The order will be ascending by default.
SELECT nameFROM customersORDER BY age;DESC
DESC will return the results in descending order.
SELECT nameFROM customersORDER BY age DESC;OFFSET
The OFFSET statement works with ORDER BY and specifies the number of rows to skip before starting to return rows from the query.
SELECT nameFROM customersORDER BY ageOFFSET 10 ROWS;FETCH
FETCH specifies the number of rows to return after the OFFSET clause has been processed. The OFFSET clause is mandatory, while the FETCH clause is optional.
SELECT nameFROM customersORDER BY ageOFFSET 10 ROWSFETCH NEXT 10 ROWS ONLY;JOINS (INNER, LEFT, RIGHT, FULL)
A JOIN clause is used to combine rows from two or more tables. The four types of JOIN are INNER, LEFT, RIGHT and FULL.
INNER JOIN
INNER JOIN selects records that have matching values in both tables.
SELECT nameFROM customersINNER JOIN ordersON customers.customer_id = orders.customer_id;LEFT JOIN
LEFT JOIN selects records from the left table that match records in the right table. In the below example the left table is customers.
SELECT nameFROM customersLEFT JOIN ordersON customers.customer_id = orders.customer_id;RIGHT JOIN
RIGHT JOIN selects records from the right table that match records in the left table. In the below example the right table is orders.
SELECT nameFROM customersRIGHT JOIN ordersON customers.customer_id = orders.customer_id;FULL JOIN
FULL JOIN selects records that have a match in the left or right table. Think of it as the “OR” JOIN compared with the “AND” JOIN (INNER JOIN).
SELECT nameFROM customersFULL OUTER JOIN ordersON customers.customer_id = orders.customer_id;EXISTS
EXISTS is used to test for the existence of any record in a subquery.
SELECT nameFROM customersWHERE EXISTS(SELECT order FROM ORDERS WHERE customer_id = 1);GRANT
GRANT gives a particular user access to database objects such as tables, views or the database itself. The below example would give SELECT and UPDATE access on the customers table to a user named “usr_bob”.
GRANT SELECT, UPDATE ON customers TO usr_bob;REVOKE
REVOKE removes a user's permissions for a particular database object.
REVOKE SELECT, UPDATE ON customers FROM usr_bob;SAVEPOINT
SAVEPOINT allows you to identify a point in a transaction to which you can later roll back. Similar to creating a backup.
SAVEPOINT SAVEPOINT_NAME;COMMIT
COMMIT is for saving every transaction to the database. A COMMIT statement will release any existing savepoints that may be in use and once the statement is issued, you cannot roll back the transaction.
DELETE FROM customersWHERE name = ‘Bob’;COMMITROLLBACK
ROLLBACK is used to undo transactions which are not saved to the database. This can only be used to undo transactions since the last COMMIT or ROLLBACK command was issued. You can also rollback to a SAVEPOINT that has been created before.
ROLLBACK TO SAVEPOINT_NAME;TRUNCATE
TRUNCATE TABLE removes all data entries from a table in a database, but keeps the table and structure in place. Similar to DELETE.
TRUNCATE TABLE customers;UNION
UNION combines multiple result-sets using two or more SELECT statements and eliminates duplicate rows.
SELECT name FROM customersUNIONSELECT name FROM orders;
UNION ALL
UNION ALL combines multiple result-sets using two or more SELECT statements and keeps duplicate rows.
SELECT name FROM customersUNIONSELECT name FROM orders;We hope this page serves as a helpful quick-reference guide to SQL commands. But if you really want to learn your SQL skills, copy-apsting code won't cut it. Check out our interactive SQL courses.
