Tag:Left table



    summary A complete SQL statement will be split into many sub clauses. During the execution of the clause, multiple temporary tables (VTS) will be generated, but only the last temporary table will be returned. Starting from this idea, we try to understand the execution process of join query and answer some common questions. Join execution […]

  • How to convert external links to internal links?


    Summary:If you can convert an outer join to an inner join, you can simplify the query optimization process. This article is shared from Huawei cloud community《Conversion of gaussdb (DWS) external connection to internal connection》, author: Wang Xiaojuan 8. In the process of query optimization, the connection order between internally connected tables can be exchanged at […]

  • MySQL’s left join pit avoidance Guide


    Original address:https://www.byteblogs.com/article/157 phenomenon left joinIn our usemysqlThe query process is very common, such as how many comments are there in an article on the blog, how many comments are there in a commodity in the mall, how many likes are there in a comment, and so on. But because ofjoin、on、whereUnfamiliar keywords, such as, can sometimes […]

  • Association table query method


    preface Imagine the scene: Xiaobai is a newcomer to the workplace. When she sees a lot of database codes, she finds that she is not clear. Xiaobai cries anxiously and goes to Baidu in a hurry. She collects a lot of data and is very confused. It’s a mess Insect: for the convenience of little […]

  • Seven join relationships in one picture


    In MySQL query, join plays an important role, so it is important to master its usage. Many students may only use a few commonly used, but to become a senior engineer is to need to master thoroughly, 360 degrees without dead angle. Picture essence Edition Text interpretation 1. You need to prepare two tables: subject […]

  • MySQL — the difference between left connection, right connection, inner connection and full connection


    Recently, it suddenly occurred to me that the database has not been used for a long time, so I want to review the knowledge about the database, so in the next month, I will mainly write the database posts, First of all, many students have the illusion that it’s enough to learn how to add, […]

  • SQL join query: the difference between left join, right join, inner join and full join


    reference resources: https://www.cnblogs.com/lijingran/p/9001302.html https://www.cnblogs.com/assasion/p/7768931.html   Because simply select * from a, B is a Cartesian product. For example, table a has 5 pieces of data and table B has 3 pieces of data, then the final result is 5 * 3 = 15 pieces of data. Therefore, two tables need to be associated: select * […]

  • Join table


    Create join Internal connectionperhapsWHERE clause Return the intersection (shadow) part of two tables. The ANSI SQL specification prefers inner join syntax: SELECT * FROM tableA INNER JOIN tableB ON tableA.id = tableB.id; SELECT * FROM tableA,tableB WHERE tableA.id = tableB.id; result: Left connection: all the records in the table a will be represented, while the […]

  • Query method of association table


    preface Imagine the scene: when Xiaobai first entered the workplace, she found that many database codes were not clear. She cried and went to Baidu in a hurry. She collected a lot of information, which was also very confusing and messy Insects: in order to facilitate the small white insects, the query methods of the […]

  • Summary of left join problems in MySQL


    Use on and where to filter table data background Left join is very common in the process of MySQL query, such as how many comments a blog article has, how many comments a commodity has in a mall, how many likes a comment has, and so on. However, due to the unfamiliarity with join, on, […]

  • SQL Server table connection


    In the database operation, the results we need may be in two or even more tables. In this case, we need to connect tables. In the multi table query, there will be connections between the fields of each table, such as the primary foreign key Association. We can use the primary foreign key to associate […]

  • Inner join on will filter out the conditions of null values on both sides


    During the work of the previous two days, I met a problem. As for the join on query, I have always been confused about the results. Here is a record. 1. First, look at the following SQL query statement: The result is 25053 2. Add a condition of o.lat = n.lat: The result is 15586 […]