如何入门SQL？

2017-09-09 本文已影响0人陈昱熹

SQL is a programming language designed to manipulate and manage data stored in relational databases.
- A relational database is a database that organizes information into one or more tables.
- A table is a collection of data organized into rows and columns.
A statement is a string of characters that the database recognizes as a valid command.
- CREATE TABLE creates a new table.
- INSERT INTO adds a new row to a table.
- SELECT queries data from a table.
- UPDATE edits a row in a table.
- ALTER TABLE changes an existing table.
- DELETE FROM deletes rows from a table.

SELECT is the clause you use every time you want to query information from a database.
WHERE is a popular command that lets you filter the results of the query based on conditions that you specify.
LIKE and BETWEEN are special operators that can be used in a WHERE clause
AND and OR are special operators that you can use with WHERE to filter the query on two or more conditions.
ORDER BY lets you sort the results of the query in either ascending or descending order.
LIMIT lets you specify the maximum number of rows that the query will return. This is especially important in large tables that have thousands or even millions of rows.

Aggregate functions combine multiple rows together to form a single value of more meaningful information.
COUNT() takes the name of a column(s) as an argument and counts the number of rows where the value(s) is not NULL.
GROUP BY is a clause used with aggregate functions to combine data from one or more columns.
SUM() takes the column name as an argument and returns the sum of all the values in that column.
MAX() takes the column name as an argument and returns the largest value in that column.
MIN() takes the column name as an argument and returns the smallest value in that column.
AVG() takes a column name as an argument and returns the average value for that column.
ROUND() takes two arguments, a column name and the number of decimal places to round the values in that column.

Primary Key is a column that serves a unique identifier for row in the table. Values in this column must be unique and cannot be NULL.
Foreign Key is a column that contains the primary key to another table in the database. It is used to identify a particular row in the referenced table.
Joins are used in SQL to combine data from multiple tables.
INNER JOIN will combine rows from different tables if the join condition is true.
LEFT OUTER JOIN will return every row in the left table, and if the join condition is not met, NULL values are used to fill in the columns from the right table.
AS is a keyword in SQL that allows you to rename a column or table in the result set using an alias.