mongodb inner join

a can be viewed as the left-hand table, B can be viewed as the right table, and left table. In other words, the records in the left table (A) will all be represented, and the right table (B) will only display records that match the search criteria (in the example: A.aid = b.bid). b tables where the records are insufficient are null. 2.right join (right join) The SQL statement is as follows: SELECT * from

A good memory is better than a bad penThere are three kinds of SQL connection: Inner connection, outer connection, cross connection.Inner connection includes: equivalent connection, non-equivalent connection, natural connectionOuter connection includes: Left join (left outer connection), right connection (right outer connection)Concrete theory, see my blog post http://blog.csdn.net/jdfkldjlkjdl/article/deta

definition: Left join (left-hand join) returns a record that includes all records in the left table and the join fields in the right table Right join (right-click join) Returns a record that includes all the records in the right table and the

SQL statement is as follows:Select * fromRight joing BOn a. Aid = B. Bid The result is as follows:Aid anum bid bname1 A11 1 B112 A22 2 B223 A33 3 B334 A44 4 b44Null null 8 b88(The number of affected rows is 5) Result description:After careful observation, we will find that the result of left join is exactly the opposite. This time, it is based on the right table (B) and is filled with null when table A is insufficient. 3.

left join is based on the left table.In other words, the records in the left table (a) are all expressed, while the right table (B) only displays records that meet the search criteria (in this example:. aid = B. bid ).All records in Table B are null. 2. Right join (right join) The SQL statement is as follows:Select * fromRight joing BOn a. Aid = B. Bid The resul

records of the left table (A) will all be represented, and the right table (B) will only display records that match the search criteria (in the example: A.aid = b.bid).The low-record of table B is null.--------------------------------------------2.right JoinThe SQL statements are as follows:SELECT * FROM ARight Join BOn a.aid = B.bidThe results are as follows:AID Anum BID bname1 a20050111 1 20060324012 a20050112 2 20060324023 a20050113 3 20060324034

right table, and B can be regarded as left table.In other words, the records of the left table (A) will all be represented, and the right table (B) will only display records that match the search criteria (in the example: A.aid = b.bid).The low-record of table B is null.--------------------------------------------2.right JoinThe SQL statements are as follows:SELECT * FROM ARight Join BOn a.aid = B.bid The results are as follows:AID Anum BID bname1 a2

statement is as follows: Select * from A Right Join b on a.aid = B.bid The results are as follows: AID anum BID bname 1 A1 NBSP;NBS　P 1 NBSP;B1 2 a2 2 NB Sp B2 3 A3 NBSP;NBSP; 3 B3 null null NBSP; 8 B8 (The number of rows affected is 4 rows)Result Description:Looking closely, you will find that the result of the left join is exactly the opposite, t

1. Noun Explanation:Cartesian product: The Cartesian product is defined as a Cartesian product of two sets X and Y in mathematics, also called a direct product, which represents x x Y, the first object is a member of X, and the second object is a member of all possible sequences of Y. Suppose set a={a,b}, set b={0,1, 2}, then two Cartesian product {(a,0), (a,1), (a,2), (b,0), (b,1), (b,2)}2. Original site reference: http://ashui.net/archives/2013/552.html1> Cross JoinSELECT * FROM table1 cross

of the left table (A) will all be represented, and the right table (B) will only display records that match the search criteria (in the example: A.aid = b.bid).The low-record of table B is null.--------------------------------------------2.right JoinThe SQL statements are as follows:SELECT * FROM ARight Join BOn a.aid = B.bidThe results are as follows:AID Anum BID bname1 a20050111 1 20060324012 a20050112 2 20060324023 a20050113 3 20060324034 a2005011

of the left table (A) will all be represented, and the right table (B) will only display records that match the search criteria (in the example: A.aid = b.bid).The low-record of table B is null.--------------------------------------------2.right JoinThe SQL statements are as follows:SELECT * FROM ARight Join BOn a.aid = B.bidThe results are as follows:AID Anum BID bname1 a20050111 1 20060324012 a20050112 2 20060324023 a20050113 3 20060324034 a2005011

regarded as left table.In other words, the records of the left table (A) will all be represented, and the right table (B) will only display records that match the search criteria (in the example: A.aid = b.bid).The low-record of table B is null.--------------------------------------------2.right JoinThe SQL statements are as follows:SELECT * FROM ARight Join BOn a.aid = B.bidThe results are as follows:AID Anum BID bname1 a20050111 1 20060324012 a2005

the records of Table A, a can be regarded as the right table, and B can be regarded as left table.In other words, the records of the left table (A) will all be represented, and the right table (B) will only display records that match the search criteria (in the example: A.aid = b.bid).The low-record of table B is null.--------------------------------------------2.right JoinThe SQL statements are as follows:SELECT * FROM ARight Join BOn a.aid = B.bid