DDL DML - CS

Online SQL

https://sql.js.org/examples/GUI/

Industry standard methods for building and modifying a database

DBMSs use a data definition language (DDL) to create, modify and remove the data structures that form a relational database. DDL statements are written as a script that uses syntax similar to a computer program.
- DML statements are written in a script that is similar to a computer program.
- These languages have different functions: DDL is used for working on the relational database structure, whereas DML is used to work with the data stored in the relational database.
DBMSs use a data manipulation language (DML) to add, modify, delete and retrieve the data stored in a relational database.
Most DBMSs use structured query language (SQL) for both data definition and data manipulation. SQL was developed in the 1970s and since then it has been adopted as an industry standard.

DBMSs use a (DDL) to create, modify and remove the data structures that form a relational database. DDL statements are written as a script that uses syntax similar to a computer program

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SQL (DDL) commands and scripts

In order to be able to understand and write SQL, you should have practical experience of writing SQL scripts.
There are many applications that allow you to do this.
For example, MySQL and SQLite are freely available ones.
When using any SQL application it is important that you check the commands available to use as these may differ slightly from those listed below.

SQL operators

Operator	Description
=	equal to
>	greater than
<	less than
>=	greater than or equal to
<=	less than equal to
<>	not equal to
BETWEEN	between a range of two values
LIKE	search for a pattern
IN	specify multiple values
AND	specify multiple conditions that must all be true
OR	specify multiple conditions where one or more conditions must be true
NOT	specify a condition that must be false

CREATE DATABASE

-- CREATE DATABASE databasename;
CREATE DATABASE School;

CREATE TABLE

-- CREATE TABLE table_name (
--     column1 datatype,
--     column2 datatype,
--     column3 datatype,
--    ....
-- );
CREATE TABLE Student (
    ID integer NOT NULL,
    LastName varchar(255) NOT NULL,
    Age integer
);

ALTER TABLE

-- ALTER TABLE table_name
-- ADD column_name datatype;
-- DROP COLUMN column_name;
ALTER TABLE Student
ADD FirstName varchar(255);
DROP COLUMN Age;

PRIMARY KEY

CREATE TABLE Student (
    ID integer NOT NULL,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Age integer,
    PRIMARY KEY (ID)
);

FOREIGN KEY ...REFERENCES

CREATE TABLE Student (
    ID integer NOT NULL,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Age integer,
    ClassID integer,
    PRIMARY KEY (ID)
    FOREIGN KEY (ClassID) REFERENCES Class(ID)
);

Data types for attributes	Description
SQL (DDL) command	Description
CREATE DATABASE	Creates a database
CREATE TABLE	Creates a table definition
ALTER TABLE	Changes the definition of a table
PRIMARY KEY	Adds a primary key to a table
FOREIGN KEY ...REFERENCES	Adds a foreign key to a table
CHARACTER	Fixed length text
VARCHAR()	Variable length text
BOOLEAN	True or False; SQL uses the integers 1 and 0
INTEGER	Whole number
REAL	Number with decimal places
DATE	A date usually formatted as YYYY-MM-DD
TIME	A time usually formatted as HH:MM:SS

SELECT FROM

SELECT LastName, Age FROM Student;

WHERE

SELECT LastName, Age FROM Student
WHERE Age > 10;

ORDER BY

SELECT LastName, Age FROM Student
WHERE Age > 10
ORDER BY Age;

GROUP BY

SELECT AVG(Age), ClassID
FROM Student
GROUP BY ClassID;

INNER JOIN

SELECT FirstName, ClassID
FROM Student
WHERE FirstName <> ""
INNER JOIN Class ON Student.ClassID = Class.ID;

AND

SELECT FirstName, ClassID
FROM Student,Class
WHERE FirstName <> ""
AND Student.ClassID = Class.ID;

SUM

SELECT SUM(Age)
FROM Student

COUNT

SELECT COUNT(*)
FROM Student

AVG

SELECT AVG(Age)
FROM Student

SQL(DML) query command	Description
SQL(DML) query command	Description
SELECT FROM	Fetches data from a database. Queries always beginwith SELECT.
WHERE	Includes only rows in a query that match a given condition
ORDER BY	Sorts the results from a query by a given column eitheralphabetically or numerically
GROUP BY	Arranges data into groups
INNER JOIN	Combines rows from different tables if the joincondition is true
SUM	Returns the sum of all the values in the column
COUNT	Counts the number of rows where the column is not NUL
AVG	Returns the average value for a column with a numericdata type
INSERT INTO	Adds new row(s) to a table
DELETE FROM	Removes row(s) from a table
UPDATE	Edits row(s) in a table

INSERT INTO

-- INSERT INTO table_name (column1, column2, column3, ...)
-- VALUES (value1, value2, value3, ...);
INSERT INTO Student (FirstName, LastName, Age, ClassID)
VALUES ("Frank", "Oldmoon", 18, 1);

DELETE FROM

-- DELETE FROM table_name WHERE condition;
DELETE FROM Student WHERE Age > 18;

UPDATE

-- UPDATE table_name
-- SET column1 = value1, column2 = value2, ...
-- WHERE condition;
UPDATE Student
SET Age = 12
WHERE LastName = "Oldmoon";