fundamentals/database/notes/05-sql-primer.md

SQL Primer

Agenda

• CRUD with SQL
• JOIN

CRUD with SQL

Create rows

Keyword: INSERT Syntax: INSERT INTO table_name (column1, column2, ...) VALUES (value1, value2, ...)

Examples

1. Insert a row with all columns

    INSERT INTO students VALUES (1, 'Tantia', 'Tope', 't@t.com', '1234567890', 1);

2. Insert a row with some columns

    INSERT INTO students (first_name, last_name) VALUES ('Tantia', 'Tope');

Read rows

Keyword: SELECT Syntax: SELECT column1, column2, ... FROM table_name WHERE condition ORDER BY column1, column2, ... ASC/DESC LIMIT #

Examples

1. Get all rows

   SELECT * FROM students;
   

2. Get certain fields from all rows

   SELECT first_name, last_name FROM students;
   

3. Filter rows by condition

   SELECT * FROM students WHERE first_name = 'Tantia';
   

4. Order rows by column

   SELECT * FROM students ORDER BY first_name ASC;
   

5. Limit number of rows

   SELECT * FROM students LIMIT 10;
   

Common operators for WHERE clause

Operator	Description	Example
=	Equal	SELECT * FROM students WHERE first_name = 'Tantia'
!= or <>	Not equal	SELECT * FROM students WHERE first_name != 'Tantia'
NOT	NOT	SELECT * FROM students WHERE NOT first_name = 'John'
>	Greater than	SELECT * FROM students WHERE iq > 150
<	Less than	SELECT * FROM students WHERE age < 100
>=	Greater than or equal	SELECT * FROM students WHERE age >= 18
<=	Less than or equal	SELECT * FROM students WHERE age <= 18
AND	AND	SELECT * FROM students WHERE first_name = 'Tantia' AND last_name = 'Tope'
OR	OR	SELECT * FROM students WHERE first_name = 'John' OR last_name = 'Mycroft'
IN	IN	SELECT * FROM students WHERE first_name IN ('John', 'Mycroft')
BETWEEN	BETWEEN	SELECT * FROM students WHERE iq BETWEEN 100 AND 150
LIKE	LIKE	SELECT * FROM students WHERE first_name LIKE '%T%'
REGEXP	REGEXP	SELECT * FROM students WHERE first_name REGEXP '^[A-Z]{1}'
NULL	NULL	SELECT * FROM students WHERE first_name IS NULL
NOT NULL	NOT NULL	SELECT * FROM students WHERE first_name IS NOT NULL

String matching wildcards

With LIKE you can use the following two wildcard characters in the pattern:

• % matches any number of characters, even zero characters.
• _ matches exactly one character.

SELECT * FROM students WHERE first_name LIKE 'T%';

SELECT * FROM students WHERE first_name LIKE 'T_';

Update rows

Keyword: UPDATE Syntax: UPDATE table_name SET column1 = value1, column2 = value2, ... WHERE condition

Examples

1. Update a row

   UPDATE students SET first_name = 'Tantia' WHERE id = 1;
   

2. Update a row with a condition

   UPDATE students SET first_name = 'Tantia' WHERE id = 1 AND first_name = 'John';
   

3. Update multiple columns

   UPDATE students SET first_name = 'Tantia', last_name = 'Tope' WHERE id = 1 AND first_name = 'John';
   

Delete rows

Keyword: DELETE Syntax: DELETE FROM table_name WHERE condition

Examples

1. Delete a row with a condition

   DELETE FROM students WHERE id = 1 AND first_name = 'John';
   

2. Delete a multiple rows

   DELETE FROM students WHERE id IN (1, 2, 3);
   

Join

Join is the widely-used clause in the SQL Server essentially to combine and retrieve data from two or more tables. In a real-world relational database, data is structured in many tables and which is why, there is a constant need to join these multiple tables based on logical relationships between them.

Inner join

Inner Join clause in SQL Server creates a new table (not physical) by combining rows that have matching values in two or more tables. This join is based on a logical relationship (or a common field) between the tables and is used to retrieve data that appears in both tables.

Assume, we have two tables, Table A and Table B, that we would like to join using SQL Inner Join. The result of this join will be a new result set that returns matching rows in both these tables. The intersection part in black below shows the data retrieved using Inner Join.

Keyword: INNER JOIN or simply JOIN Syntax: SELECT column1, column2, ... FROM table_name1 JOIN table_name2 ON condition

For example, we want to get the batch names of all the students along with their names.

id	first_name	last_name	batch_name
1	John	Watson	Sherlock
2	Mycroft	Holmes	Sherlock

This can be achieved by using the following SQL query:

SELECT s.first_name, s.last_name, b.batch_name FROM students s JOIN batches ON s.batch_id = b.id;