SQL 基础

SQL 常用代码

• 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.

（选择全部）SELECT * from celebs;
（创建表格）CREATE TABLE celebs (id INTEGER, name TEXT, age INTEGER);
（插入行数据）INSERT INTO celebs (id, name, age) VALUES (1, 'Justin Bieber', 21);
（选择某列）SELECT name FROM celebs;
（更新信息）UPDATE celebs SET age = 22 WHERE id =1;
（添加表列）ALTER TABLE celebs ADD COLUMN twitter_handle TEXT;
（处理缺失值）DELETE FROM celebs WHERE twitter_handle IS NULL;

• 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.

（多变量筛选）SELECT name ,imdb_rating FROM movies;
（唯一筛选）SELECT DISTINCT genre FROM movies;
（条件匹配）SELECT * FROM movies WHERE imdb_rating > 8;
（文本部分匹配_单）SELECT * FROM movies WHERE name LIKE 'Se_en';
（文本部分匹配_多）SELECT * FROM movies WHERE name LIKE 'a%' ;
（范围内的数据）SELECT *FROM movies WHERE name BETWEEN 'A' AND 'J';
（与条件选择）SELECT * FROM movies WHERE year BETWEEN 1990 AND 2000 AND genre = 'comedy';
（或条件选择）SELECT * FROM movies WHERE genre = 'comedy' OR year < 1980;
（排序）SELECT * FROM movies ORDER BY imdb_rating DESC;
（选取前几个）SELECT * FROM movies ORDER BY imdb_rating ASC LIMIT 3;

• 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.

（计数）SELECT COUNT(*) FROM fake_apps;
（条件计数）SELECT COUNT(*) FROM fake_apps WHERE price = 0;
（分组计数）SELECT price ,COUNT(*) FROM fake_apps GROUP BY price;
（求和）SELECT SUM(downloads) FROM fake_apps;
（最大值）SELECT MAX(downloads) FROM fake_apps;
（最小值）SELECT MIN(downloads) FROM fake_apps;
（平均值）SELECT AVG(downloads) FROM fake_apps ;
（控制小数点）SELECT price , ROUND(AVG(downloads),2) FROM fake_apps GROUP BY price;

• 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.

（创建主键表格）CREATE TABLE artists(id INTEGER  PRIMARY KEY ,name TEXT);
（外部健）SELECT * FROM albums WHERE artist_id =3;
（合并表格）SELECT albums.name , albums.year , artists.name  FROM albums , artists ;
（JOIN合并）SELECT * FROM albums JOIN artists ON albums.artist_id = artists.id;
（左合并）SELECT * FROM albums LEFT JOIN artists ON albums.artist_id = artists.id ;
（重命名）SELECT albums.name AS 'Album' , albums.year , artists.name AS 'Artist'  FROM albums  JOIN artists ON albums.artist_id = artists.id  WHERE albums.year > 1980;

• 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 theleft 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.**