Left join, join, right join difference, leftjoin

Left join, join, right join difference, leftjoin
Left join returns records that include all records in the left table and join fields in the right table.Right join returns records that include all records in the right table and the joined fields in the left table.Inner join (equivalent join) returns only rows with equal join fields in two tables. Example:--------------------------------------------Table A records the following:aID　　　　　aNum1　　　　　a200501112　　　　　a200501123　　　　　a200501134　　　　　a200501145　　　　　a20050115 Table B records the following:bID　　　　　bName1　　　　　20060324012　　　　　20060324023　　　　　20060324034　　　　　20060324048　　　　　2006032408 --------------------------------------------1.left joinThe SQL statement is as follows:select * from Aleft join B on A.aID = B.bID The result is as follows:aID　　　　　aNum　　　　　bID　　　　　bName1　　　　　a20050111　　　　1　　　　　20060324012　　　　　a20050112　　　　2　　　　　20060324023　　　　　a20050113　　　　3　　　　　20060324034　　　　　a20050114　　　　4　　　　　20060324045　　　　　a20050115　　　　NULL　　　　　NULL (The number of affected rows is 5)Result description:Left join is based on the records of table A. A can be seen as the left table, B can be seen as the right table, and 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 joinThe SQL statement is as follows:select * from Aright join B on A.aID = B.bID The result is as follows:aID　　　　　aNum　　　　　bID　　　　　bName1　　　　　a20050111　　　　1　　　　　20060324012　　　　　a20050112　　　　2　　　　　20060324023　　　　　a20050113　　　　3　　　　　20060324034　　　　　a20050114　　　　4　　　　　2006032404NULL　　　　　NULL　　　　　8　　　　　2006032408 (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.inner joinThe SQL statement is as follows:select * from Ainnerjoin B on A.aID = B.bID The result is as follows:aID　　　　　aNum　　　　　bID　　　　　bName1　　　　　a20050111　　　　1　　　　　20060324012　　　　　a20050112　　　　2　　　　　20060324023　　　　　a20050113　　　　3　　　　　20060324034　　　　　a20050114　　　　4　　　　　2006032404 Result description:Obviously, only the records of A. aID = B. bID are displayed here. This indicates that inner join is not based on WHO, and only records meeting the conditions are displayed.--------------------------------------------Note:The left join operation is used to combine records of the source table in any FROM clause. Use the left join operation to create a LEFT Outer JOIN. The outer join on the left contains all records from the first (left) two tables, even if there is no record with consistent values in the second (right) table. Syntax: FROM table1 left join table2 ON table1.field1 compopr table2.field2 Description: The table1 and table2 parameters are used to specify the names of the tables to be combined.The field1 and field2 Parameters specify the names of joined fields. These fields must have the same data type and contain the same data type, but they do not need the same name.The compopr parameter specifies the relational comparison operator: "=", "<", ">", "<=", ">=" or" <> ".If you want to JOIN a field that contains the Memo data type or OLE Object data type in the inner join operation, an error will occur.