Database 02

Hello 👋🏾 New blog post new dataday post... it's all about SQL queries

📓 All the examples below are based on this table ⬇️.

📝 Statement is the query.

SELECT

• this statement is used to select data from a database.
• Select DISTINCT is used to return only different values.

Example

Select * from Customers;
#returns all the table 

Select DISTINCT Country from Customers;
#returns Germany, UK, South Korea, Malaysia

WHERE clause

• This clause is used to filter records.
• And is used to extract only those records that a specified condition.
• Operators in WHERE clause (= , > , <, != , BETWEEN, LIKE, IN).

Example

SELECT * FROM Customers WHERE Country = 'UK'

SQL AND, OR and NOT

• AND displays a record if all conditions are TRUE.
• OR displays a record if any of the conditions separated by OR is TRUE.
• NOT displays a record if the condition(s) is NOT TRUE.

Example

SELECT * FROM Customers WHERE lastname = "Doe" AND (Country= 'Germany' OR 'South Korea');

SQL ORDER BY keyword

• This one is used to sort the result in ascending or descending order, by default the records are ascending.

INSERT INTO

• This statement is used to insert a new row into a table.

Example

INSERT INTO Customers (firstname, lastname, country) VALUES ('james', 'bond', 'Finland')

SQL Null Values

• first Null value is equal to no value.
  • It's not like zero value, it is a value that has been left blank during record creation.

Example

SELECT firstname FROM Customers WHERE firstname IS NULL;
#this query will not return anything.

SQL UPDATE

• update is to change existing records in a table.
• ⚠️ if you omit WHERE clause, All records will be updated

Example

UPDATE Customers SET firstname= 'Amy', lastname = 'Alex' WHERE ID=2;
#this will change Alice Smith to Amy Alex

SQL DELETE

• delete statement is used to delete existing records in a table.

Example

DELETE FROM Customers WHERE ID=2;
#this will delete id 2.

SQL SELECT TOP

• select top is used to specify the number of records to return.
• useful on large tables.

Example

#sql server
SELECT TOP 3 * FROM Customers;

#MYSQL
SELECT * FROM Customers Limit 3;

#oracle
SELECT * FROM Customers FETCH FIRST 3 ROWS ONLY;

I hope you enjoyed reading :)

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