The first thing i want tell about the first link i found from google. It make by qoura. Fine, i like it: https://www.quora.com/How-do-I-learn-more-advanced-SQL
Firstly i want learn SQL from basic, because maybe i’m a beginer in every thing. Of course, i dont like this. But some day, when i joined my project, i know i am a chicken. So, if i’m not a bigger, i will be a person.
First ez query:
SELECT * FROM Customers;
This query will select all record with all attributes of Customers.
SELECT DISTINCT
The SQL select distinct statement is used to return only distinct(diffirent) values. Inside a table, a column often contains many duplicate values, and sometimes you only want to list the different (distinct) value.
Syntax:
SELECT DISTINCT column1, column2,..
FROM table_name;
Example:
SELECT DISTINCT Country FROM Customers;
This query will have result all different Country of customer.
WHERE Clause
The where clause is used to filter records. The where clause is used to extract only those records that fulfill a specified condition (Đáp ứng một điều kiện cụ thể). Syntax:
SELECT column1, column2
FROM table_name
WHERE condition;
This query will return the records of column1 and column2 with condition is condition
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SQL ORDER BY
The order by key word is used to sort the result-set in ascending or desending order. In the default, the order by will sort the records in ascending order by default. To sort records desending, use DESC
SELECT column1, column2, ...
FROM table_name
ORDER BY column1, column2, ... ASC|DESC;
INSERT INTO
Used to insert new records in table.
INSERT INTO table_name (column1, column2, column3, ...)
VALUES (value1, value2, value3, ...);
ex:
INSERT INTO Customers (CustomerName, ContactName, Address, City, PostalCode, Country)
VALUES ('Cardinal', 'Tom B. Erichsen', 'Skagen 21', 'Stavanger', '4006', 'Norway');
SQL NULL VALUE
What is null value? A field wit null value is field with no vale.
IS NULL syntax
SELECT column_names
FROM table_name
WHERE column_name IS NULL;
IS NOT NULL syntax
SELECT column_names
FROM table_name
WHERE column_name IS NOT NULL;
