postgresql left join

Left Join, leftjoin After the development has executed a statement for more than two hours, the developer asked me why I have been running it for so long.Statement format:select * from tgt1 a left join tgt2 b on a.id=b.id and a.id>=6 order by a.id;This is a typical misunderstanding. The intention is to filter Table[]

Document directory TIPS: Case Analysis User table:Id | Name --------- 1 | libk 2 | zyfon 3 | daodao User_action table:User_id | action --------------- 1 | jump 1 | kick 1 | jump 2 | run 4 | swim SQL:Select ID, name, action from user as uLeft join user_action A on u. ID = A. user_id Result:Id | Name | action--------------------------------1 | libk | jump ①1 | libk | kick ②1 | libk | jump ③2 | zyfon | run ④3 | daodao | null ⑤ Analysis: Note that use

SQL LEFT join instance and syntax usage Left joins and similar rights join operations can be used in any FROM clause to combine records from two tables. The left JOIN operation is used to create a

Tags: and difficult to affect its causes cannot div save exampleObjectiveIn this section we analyze the left join and not EXISTS, short content, in-depth understanding, always to review the basics.Left JOIN ... Is null and NOT EXISTS analysisWe have previously analyzed that in query processing null value is based on the three-value logic, as long as there is null

The following articles mainly describe the Efficiency Analysis of MySQL left join joint queries. I found an analysis on the efficiency of MySQL left join joint queries on a website with good reputation, I hope to help you in this regard. User table: Code: Id | name 1 | libk 2 | zyfon 3 | daodao User_action table:

SQL join usage (full, left, out, inner) A. Cross join Cartesian product if there is no where Condition Clause, it will return the Cartesian product of the two joined tables, and the number of rows returned is equal to the product of the number of rows of the two tables;For example, the execution results of A, B, and C are the same but the effic

There are two tables, a and B. The first two fields are identical: (id int, name varchar (10 )...)Id name---------------------12 B3 c Do you know the running result of the following query statement? :1.Select * from a left join B on a. id = B. id where a. id = 12.Select * from a left join B on a. id = B. id and a. id =

User table:ID | Name———1 | Libk2 | Zyfon3 | DaodaoUser_action table:user_id | Action—————1 | Jump1 | Kick1 | Jump2 | Run4 | SwimSql:Select ID, name, action from user as ULeft join user_action A on u.id = a.user_idResultID | name | Action——————————–1 | LIBK | Jump①1 | LIBK | Kick②1 | LIBK | Jump③2 | Zyfon | Run④3 | Daodao | Null⑤Analysis:Notice that there is also a user_id=4, Action=swim record in the user_action, but it did not appear in the results,I

Tags: query font condition Auto Compare Name table between title testThese two conditions placed in different places is very easy to confuse people, so often query out the inexplicable results, especially when the condition of the replica and the main table does not match, the following with a B table for example, I understand the simple. The first thing to understand is: The condition behind on is that the data that participates in the left

Label:The EFT Join left connection is based on left table, showing all the coordinates of the row, the right table and the left table associated with the data will be displayed, not associated is not displayed. The keyword is left join