A complete collection of SQL commands. Each command has an example. Xiaobai can become a God after reading it!

Hello everyone , This is the network technology dry goods circle , What I bring to you today is SQL The list of commands , Each command comes with an example , about sql It is undoubtedly a blessing for beginners and even Xiaobai ！

SELECT

SELECT Probably the most commonly used SQL sentence , Each use SQL When querying data , Almost always use it .

for example , In the following code , from customers Query in table name Field .

SELECT name
FROM customers;

SELECT *

Use * Represents all columns in the query table

SELECT * FROM customers;

SELECT DISTINCT

SELECT DISTINCT Return only different data , It means that if there is a duplicate record , Only one of the duplicate records will be returned .

SELECT DISTINCT name
FROM customers;

SELECT INTO

SELECT INTO Copy the specified data from one table to another .

SELECT * INTO customers
FROM customers_bakcup;

SELECT TOP

SELECT TOP Only the highest... In the table is returned x Number or percentage .

The following code will return customers The first in the table 50 results ：

SELECT TOP 50 * FROM customers;

The following code will return customers Front of table 50%

SELECT TOP 50 PERCENT * FROM customers;

AS

as Renaming is to give an alias to a related column , for example , In the following code , We will name Rename the column to first_name：

SELECT name AS first_name
FROM customers;

FROM

FROM Specify the source table of the query

SELECT name
FROM customers;

WHERE

Filter query , Returns the result of the matching condition , General conditions will match =,>,<,>=,<= And so on

SELECT name
FROM customers
WHERE name = ‘Bob’;

AND

AND Combine two or more conditions in a single query , All conditions must be met to return a result .

SELECT name
FROM customers
WHERE name = ‘Bob’ AND age = 55;

OR

OR Combine two or more conditions in a single query , As long as one of the conditions is met, the result can be returned .

SELECT name
FROM customers
WHERE name = ‘Bob’ OR age = 55;

BETWEEN

BETWEEN Filter the values in the specified range

SELECT name
FROM customers
WHERE age BETWEEN 45 AND 55;

LIKE

like For fuzzy query , In the following example code , Will return a name that contains characters Bob The data of

SELECT name
FROM customers
WHERE name LIKE ‘%Bob%’;

LIKE Other operators of ：

• %x — All will be selected to x Initial value
• %x% — Will select include x All values
• x% — All will be selected to x Ending value
• x%y — All will be selected to x Start with y Ending value
• _x% — All with will be selected x As the value of the second character
• x_%— All will be selected to x A value that begins with a length of at least two characters , You can add additional _ Characters to extend the length requirement , namely x___%

IN

IN Allow us to use WHERE Command to specify multiple values to select .

SELECT name
FROM customers
WHERE name IN (‘Bob’, ‘Fred’, ‘Harry’);

IS NULL

IS NULL Will only return with NULL Row of values .

SELECT name
FROM customers
WHERE name IS NULL;

IS NOT NULL

IS NOT NULL On the contrary —— It will only return no NULL Row of values .

SELECT name
FROM customers
WHERE name IS NOT NULL;

CREATE

CREATE Can be used to create a database 、 surface 、 Index or view .

CREATE DATABASE

CREATE DATABASE Create a new database .

CREATE DATABASE dataquestDB;

CREATE TABLE

CREATE TABLE Create a new table in the database .

CREATE TABLE customers (
    customer_id int,
    name varchar(255),
    age int
);

CREATE INDEX

CREATE INDEX Generate indexes for tables , Indexes are used to retrieve data from the database faster .

CREATE INDEX idx_name
ON customers (name);

CREATE VIEW

CREATE VIEW according to SQL Statement to create a virtual table , A view is like an ordinary table （ It can be queried like a table ）, But it doesn't Not saved as a permanent table in the database .

CREATE VIEW [Bob Customers] AS
SELECT name, age
FROM customers
WHERE name = ‘Bob’;

DROP

DROP Statement can be used to delete the entire database 、 A table or index .

It goes without saying ,DROP The command should only be used when absolutely necessary .

DROP DATABASE

DROP DATABASE Delete the entire database , Include all its tables 、 Index, etc. and all the data in it .

Use this command with extreme caution ！

DROP DATABASE dataquestDB;

DROP TABLE

DROP TABLE Delete a table and its data .

DROP TABLE customers;

DROP INDEX

DROP INDEX Delete the index in the database .

DROP INDEX idx_name;

UPDATE

UPDATE Statement is used to update data in a table , for example , The following code will customers The name in the table is Bob Change your age to 56.

UPDATE customers
SET age = 56
WHERE name = ‘Bob’;

DELETE

DELETE You can delete all rows in the table （ Use *）, Can also be used as WHERE Clause to delete rows that meet certain conditions .

DELETE FROM customers
WHERE name = ‘Bob’;

ALTER TABLE

ALTER TABLE Allows you to add or remove columns from the table .

by customers Add new column to table surname

ALTER TABLE customers
ADD surname varchar(255);

Delete customers In the table surname Column

ALTER TABLE customers
DROP 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 following code , We use *, therefore customers The total number of rows to be returned .

SELECT COUNT(*)
FROM customers;

SUM

SUM Returns the sum of numeric columns .

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

GROUP BY Statement groups rows with the same value into a summary row , This statement is usually used with aggregate functions .

for example , The following code will show us customers The average age of each name in the table .

SELECT name, AVG(age)
FROM customers
GROUP BY name;

HAVING

HAVING Execution and WHERE Clause . The difference is HAVING Used to aggregate functions .

The following example returns the number of rows per name , But only for those with 2 Names of more than records .

SELECT COUNT(customer_id), name
FROM customers
GROUP BY name
HAVING COUNT(customer_id) > 2;

ORDER BY

ORDER BY sets the order of the returned results. The order will be ascending by default.

SELECT name
FROM customers
ORDER BY age;

DESC

DESC Results will be returned in descending order .

SELECT name
FROM customers
ORDER BY age DESC;

OFFSET

OFFSET Statements and ORDER BY Use it together , And specify the number of rows to skip before starting to return rows from the query .

SELECT name
FROM customers
ORDER BY age
OFFSET 10 ROWS;

FETCH

FETCH Specifies that after processing OFFSET Number of rows to return after clause .

OFFSET The clause is mandatory , and FETCH Clauses are optional .

SELECT name
FROM customers
ORDER BY age
OFFSET 10 ROWS
FETCH NEXT 10 ROWS ONLY;

Connect ( Inside 、 Left 、 Right 、 whole )

JOIN Clause is used to combine rows from two or more tables ,JOIN The four types of are INNER、LEFT、RIGHT and FULL.

INNER JOIN

INNER JOIN Select records with matching values in both tables .

SELECT name
FROM customers
INNER JOIN orders
ON customers.customer_id = orders.customer_id;

LEFT JOIN

LEFT JOIN Select a record from the left table that matches the record in the right table , In the following example , The left table is customers.

SELECT name
FROM customers
LEFT JOIN orders
ON customers.customer_id = orders.customer_id;

RIGHT JOIN

RIGHT JOIN Select the record matching the record in the left table from the right table , In the following example , The right table is orders.

SELECT name
FROM customers
RIGHT JOIN orders
ON customers.customer_id = orders.customer_id;

FULL JOIN

FULL JOIN Select the matching records in the left table or the right table .

And “AND”JOIN（INNER JOIN） comparison , Think of it as “OR”JOIN.

SELECT name
FROM customers
FULL OUTER JOIN orders
ON customers.customer_id = orders.customer_id;

EXISTS

EXISTS Used to test whether there are any records in the subquery .

SELECT name
FROM customers
WHERE EXISTS
(SELECT order FROM ORDERS WHERE customer_id = 1);

GRANT

GRANT Allow specific users to access database objects , Such as table 、 View or database itself .

The following example will be named “usr_bob” The user assigned to customers Tabular SELECT and UPDATE Access right .

GRANT SELECT, UPDATE ON customers TO usr_bob;

REVOKE

REVOKE Delete the user's permissions on specific database objects .

REVOKE SELECT, UPDATE ON customers FROM usr_bob;

SAVEPOINT

SAVEPOINT Allows you to identify a point in a transaction , You can roll back to this point later , Similar to creating a backup .

SAVEPOINT SAVEPOINT_NAME;

COMMIT

COMMIT Used to save each transaction to the database ,COMMIT Statement will release any existing savepoints that may be in use , And once the statement is issued , The transaction cannot be rolled back .

DELETE FROM customers
WHERE name = ‘Bob’;
COMMIT;

ROLLBACK

ROLLBACK Used to undo transactions not saved to the database , This can only be used to undo the last issue COMMIT or ROLLBACK The business since the order , You can also rollback to a previously created SAVEPOINT.

ROLLBACK TO SAVEPOINT_NAME;

TRUNCATE

TRUNCATE TABLE Delete all data entries from the tables in the database , But keep the table and structure .

TRUNCATE TABLE customers;

UNION

UNION Use two or more SELECT Statement to combine multiple result sets and eliminate duplicate rows .

SELECT name FROM customers
UNION
SELECT name FROM orders;

UNION ALL

UNION ALL Use two or more SELECT Statement to combine multiple result sets and keep duplicate rows .

SELECT name FROM customers
UNION ALL
SELECT name FROM orders;