事情是这样的,去年入职的新公司,之后在代码review的时候被提出说,不要写join,join耗性能还是慢来着,当时也是真的没有多想,那就写in好了,最近发现in的数据量过大的时候会导致sql慢,甚至sql太长,直接报错了。 这次来浅究一下,到底是in好还是join好,仅目前认知探寻,有不对之处欢迎指正。 以下实验仅在本机电脑...
https://github.com/javastacks/spring-boot-best-practice 二、先来试少量数据的情况 用户表插一千条随机生成的数据,订单表插一百条随机数据 查下所有的订单以及订单对应的用户 下面从三个维度来看 多表连接查询成本 = 一次驱动表成本 + 从驱动表查出的记录数 * 一次被驱动表的成本 1、join JOIN: explain form...
https://github.com/javastacks/spring-boot-best-practice 二、先来试少量数据的情况 用户表插一千条随机生成的数据,订单表插一百条随机数据 查下所有的订单以及订单对应的用户 下面从三个维度来看 多表连接查询成本 = 一次驱动表成本 + 从驱动表查出的记录数 * 一次被驱动表的成本 1、join JOIN: explain form...
INSERT INTO question_practice_detail VALUES(2,3214,112,'wrong'); INSERT INTO question_practice_detail VALUES(3,3214,113,'wrong'); INSERT INTO question_practice_detail VALUES(4,6543,111,'right'); INSERT INTO question_practice_detail VALUES(5,2315,115,'right'); INSERT INTO question_practice_d...
resultFROMquestion_practice_detailWHEREdevice_idIN(SELECTdevice_idFROMuser_profileWHEREuniversity='浙江大学'); 普通查询: SELECTa.device_id, question_id, resultFROMquestion_practice_detail a, user_profile bWHEREa.device_id=b.device_idandb.university='浙江大学'ORDERBYquestion_idASC; ...
In practice, it’s the LEFT JOIN command that’s most often used, because it’s so obvious, as we’ll see in an example below. Learn all about SQL joins SQL JOIN: The LEFT JOIN The LEFT JOIN SQL command is the most commonly used external join command. It lists all elements commo...
Using JOIN in SQL doesn’t mean you can only join two tables. You can join 3, 4, or even more! The possibilities are limitless. The best way to practice SQL JOINs is LearnSQL.com's interactiveSQL JOINscourse. It contains over 90 hands-on exercises that let you refresh your SQL JOINs...
It's a better programming practice to specify your JOIN conditions in the FROM clause. When joining tables you'll have to make sure that there is no ambiguity in the column names. In our example both Manufacturer and Product tables have a column named ManufacturerID, that's why we prefixed...
In SQL, a self join can be used in connection with one or more separate tables. While this is not a clean self join, it is quite frequent in practice.A real-world example is flight information in airports, which generates massive amounts of data each hour. Assume we want to look for...
Now it’s your turn to practice using outer joins. Try the following exercises. Exercise 1: left outer join Write a query to list all products and their associated order items, including products that have never been ordered. Write your answer here! Run QueryShow Answer Exercise 2: full out...