sql join 2 tables

the configuration class that can be used to set the key value of the sort group.4. SummaryTo summarize, the core of join is the small table memory can be put down, can put down the map joinNot fit, reduce join.Reduce joins a large amount of network and disk IO, with poor performance and a way to optimize:　　　　　method One : Semi-join, map filter join demand column

Left join returns records that include all the records in the left table and the equivalent of the junction fields in the right tableRight join returns records that include all records in the right table and the junction fields in the left tableINNER JOIN (equivalent join) returns only rows that have the same

How can we query the information of different tables together when we talk about the inefficiency of the related sub-queries in the previous blog post? This requires a table join.Unlike the previous union query, the Union combines different tables, that is, vertical joins, which are spelled upright.A table join is a horizontal

code Create Clustered Index GT On Goodstype (ID) Create Clustered Index G On Goods (good_type) Set Statistics Profile On Select * From Goods As G Inner Join Goodstype As GT On G. good_type = GT. ID Option (Merge Join ) Result: Note:1> after an index is created, the execution of merge join has no sorting overhead.

organized. It means: ISelect a. a, a. BMediumA. a is the meaning of field a in table. IISelect and from are the fields to be queried in table B of Table. 3.From a left out join BThat is, from table a to table B on the left link,The matching condition is that the field of Table a is equal to the c field of Table B. I believe that everyone understands 80% through these explanations. Let's explain it to you through an example: Create two

is not satisfied, because the supplier code is really meaningless to humans, join can help at this time. By joining the suppliers table, we can query the Supplier name. Select productid, productname, suppliers. supplierid, suppliers. companyName from products inner join suppliers on products. supplierid = suppliers. supplierid order by productname Outer Join Th

The left join is a left table datum, 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 on. * * Basic usage is as follows:Select Table Left Join Table on = b.ta_id* *Note: 1?? Where on the associated field should be the same field (the f

Tags: oracle merge sort Jo Use_mergeSort merge joins (sort merge join) are two tables that use the sort operation (sort) and merge operation (merge) to get the connection method of the result set when making a connection.The pros and cons of a sort merge connection and the scenarios that apply are as follows:A, typically, a sort merge connection performs much less efficiently than a hash connection, but the

to be before the value list, while the record of the table on the right of join is behind the value list. For example, A join B ON (. id = B. id). If table A and table B both have 1 record with id = 3, the key combination of the record in Table A is ), the key combination for this record in Table B is (3, 1 ). During sorting, you can ensure that the records of Table A are before the records of Table B. In

ca 15000 berkeley ca 36000 mit ma 10000 cornell TD valign= "Top" width= "133" >ny 21000 harvard ma 29000 Table 1 data in the College tableAs shown in 1, we have queried the school information contained in the Apply table, as Harvard has not been queried, so we know that there are no students to apply for Harvard.An inner join