Tag：Descartes

Time：202192
Suppose there are the following two tables: A： id age 1 11 2 12 3 13 4 14 B: id name 2 aaa 3 bbb 4 ccc 5 ddd Inner join There is only one application scenario, inner join or join, which is equivalent to inner join select a.*,b.* from a inner join(join) b on a.id […]

Time：202175
There are four ways to join Oracle tables Sort merge join Nested loops join Hash join Cartesian product Sort merge join Sort merge join is to sort two connected tables with join columns, merge the sorted result set, and then get matching records. If there is an index on the join column to avoid sorting, […]

Time：202134
1. Archimedes spiral Archimedes spiral is also known as “constant velocity spiral”. When a point P moves along the moving ray OP at the same speed, the ray rotates around the point o at the same angular speed. The trajectory of point P is called “Archimedean spiral”. The Cartesian coordinate equation of Archimedes spiral is […]

Time：202133
1. Permutation problem You need to use Python’s itertools module import itertools a=[1,2,3] #Take, arrange (mathematical formula: A32) for i in itertools.permutations (a, 2): # 2 is taken twice. A can be a string or a list print(i) ”’ (1, 2) (1, 3) (2, 1) (2, 3) (3, 1) (3, 2) ”’ #Combination (mathematical formula: […]

Time：20201026
https://v.youku.com/v_show/id… In the last issue, we introduced the complex query function. In this issue, we will continue to explain the tables with different splitting rules. Now I have four nodes, four nodes are divided by modulus, TB_ There are five pieces of data on mod. There is a bracket for 67, which means that if […]

Time：20201024
Three level model structure Mode, external mode (sub mode), internal mode Two level mapping External schema / schema mapping → guarantee logical independence of data Internal mode / mode influence → guarantee the physical independence of data Introduction to relational database E.f.codd of IBM is the most systematic and strict method to put forward relational […]

Time：20201012
https://v.youku.com/v_show/id… Hello, everyone. In the past, we introduced the use of explain. In this issue, we will introduce the explain function in detail. Explain can be used to analyze the distributed query plan and see the results of the distributed SQL. In addition to this function, we can also do some SQL tuning. Let’s take […]

Time：2020215
1 – execution sequencefromonjoinwheregroup byhavingselectdistinctorder bylimit From (the Cartesian product is calculated for the tables on the left and right sides of from to get the virtual table V1)On (filter V1 table to get V2)Join (carry out left join, right join and other processing on V2 table to get V3)Where (where filter V3 to get […]

Time：20191125
1. internal connection Rule: return common records of two tables Syntax: — grammar 1 Select * from table 1 inner table 2 on table 1. Public field = Table 2. Public field — grammar two Select * from table 1, table 2 where table 1. Public field = Table 2. Public field […]

Time：20191013
7. Multitable query For queries, we have learned simple queries, limited queries and query ranking before, which belong to the standard statements of SQL. The main function of the singleline function in the previous chapter is to make up for the deficiencies of queries. From the beginning of multitable query, it has formally entered the […]

Time：201994
Connections in SQL One of the core of relational database is connection. In different standards, the connection may be written differently. The two most important SQL standards are SQL92 and SQL99. The following figures indicate the time when the standard was put forward. Connections in SQL92 The table used in the case is player table, […]