sql joins explained

included in the two tables. If the tuple is matched on the join condition, the other table returns the corresponding, and no returns NULL.Seven, cross-linkingCross-connect Corss join, used to combine all records from the first table with all records of the second table once and back, is useful when generating a test database, for example, you define 7 last names, 7 names, and then cross-connect to generate 49 records.If you have the following table: Last Name table　　　　Name table:　　　　Execute the

Intersect is as follows: [SQL statement 1]Intersect[SQL Statement 2] The Intersect operator derives a result table by including only the rows in Table1 and table2 and eliminates all duplicate rows. When all is used with intersect (intersect all), duplicate rows are not eliminated.Note: Several query result rows that use an operator must be consistent. Select Date from Store_informationIntersectSelect

, therefore, is also called an inline view. When you use a query in the FROM clause, you must specify an alias for the subquery. 3, paging query1) Top N: First N records. Select Top 5 * from emp ORDER by HireDate Displays information from 5th to 9th persons (by Sal High and low) Select Top 5 from EMP where empno not in (select Top 4 empno from emp ORDER BY Sal Desc) Order by Sal Desc Identity: Indicates that the field is growing from "1", with "1" added e

Label:1. Overlay index: The "Nonclustered index column" should exist in the result set contained in select and where, so that the index table can be done without looking up the base table; 2. Index Cross: is for an extension of the overlay index (here the nonclustered index is built in L1 and L2, select * from t _ Where l1= ' asd ' and l2= ' qwe '), then two nonclustered index tables are join;3. Index Connection: is a special case of index crossover (select L1,l2 from T_ where l1= ' ASD ' and l2

Tags: data Select right outer connection out SEL outside connection left outer connection outer keywordjoin is divided into: inner join (INNER join), outer join (OUTER join). Among them, the outer join is divided into: left outer connection (OUTER join), right outer join (OUTER join), full outer join (complete OUTER join), where the "OUTER" keyword of the outer connection can be omitted from writing.Example: table A has column ID, the value is: 1 2 3 4Table B has column IDs, with values of: 3 4

Label:There are probably a few joinsin SQL : Cross Join INNER JOIN Left OUTER JOIN Right outer join Full OUTER JOIN The first is based on the cross join ( Cartesian product), then the inner join, and the rows in the result set of the Cartesian product that do not conform to the join condition are removed. The left outer join is added to the result set of the inner join and the rows that are not selected in the leftmost table are filled with NUll for

. Equivalent (same as the following execution effect)A:select a.*,b.* from table1 a,table2 b where a.id=b.idB:select * FROM table1 Cross join Table2 where table1.id=table2.id (note: The Add condition after cross join can only be used where, cannot be used)Three, cross-connect (full)1. Concept: A cross join without a WHERE clause will produce a Cartesian product of the table involved in the join. The number of rows in the first table multiplied by the number of rows in the second table equals the

, Route R2 2 WHERE R1.num= and R1.company=R2.companyOur route table uses fields (num, company) to self-connect. What does the result mean?You can get to know any two connecting stations of each bus line.Second, use the Stop field to self-connect the route (bus Line table).1 SELECT * from Route R1, Route R2 2 WHERE R1.stop=r2.stop;The result of the query is that all bus lines are shared with the same station. Does this result make a lot of sense to the transfer?From these two examples we can see

salary of all employees above the department 20Select Ename,sal,deptno from emp where sal > All (select Sal from emp where deptno=20)> is a single-line operator, and the subquery is multiline, with all to resolve the syntax conflict, to meet the ' all ' requirementAttention:1. Cannot write order by in subquery2. Multiline subqueries cannot be matched by single-line operators3. Single-line subqueries can match multiple line operators4. Subqueries must be written in parentheses5. Sub-query to be

Label:Test platform: Ms-sql R2;The following two t_a and T_b tables are tested for joins and all-First: Create a tableCREATE table T_a ([aid] [Int],[anum] [char])Second: Create B TableCREATE TABLE T_b ([bid] [int],[aname] [char])Third: 1: Add data to Table AInsert into T_a (aid,anum) SELECT 1,20050111 unionselect 2,20050112 unionselect 3,20050113 unionselect 4,20050114union All SELECT 5,20050115A table da

Tags: happy sharing feature for me select information from where BSPToday's work learning path is a database of small knowledge, at that time did not distinguish the why, hereby recorded share a sudden. As we all know, the tables of the database are all separate, but when we do a union query (multi-table query), we get the value returned by the database as if it were in a table, because the database generates a temporary table to return to the data information we want when we make a federated qu

Label:Paginated SQL statements for multi-table joins : originally Select Top page size a.aid from table 1 a LEFT JOIN table 2 B on a.aid=b.aid where A.aid not in (select Top page index * page size love from Table 1 a LEFT JOIN table 2 B on a.aid=b.aid where condition) and condition; after optimization With C as (select Rn=row_number () over (order by x.id), * FROM (SELECT * FROM table 1 a LEFT JOIN table 2

times times smaller, here is the experience)3. Large table to large table (Sort Merge Join)The two tables were re-shuffle by the join keys to ensure that records with the same join keys value were divided into the corresponding partitions. After partitioning, the data within each partition is sorted, sorted, and then connected to the records in the corresponding partitionBecause two sequences are ordered, traverse from the beginning, hit the same key on the output, if different, the left side o

To select data from two or more tables, we typically use table joins to implement this functionality. This section describes the concept of join (join). For this we prepared two test tables: album (album table) and track (track table). Album table: Contains an overview of 200 music CDs from Amazon. Album (ASIN, title, artist, Price, release, label, Rank) Track Table: The details of each album's tracks (because they are music CDs, so can also be ca

(T.taskid,'__')= IsNull(N.taskid,'__')Group byT.taskidThe execution plan is as follows: (this looks much simpler than the above, imagine that performance will improve)When the amount of data is large, the performance aspect of using the left join will be greatly improved, note that when on, the null value is converted using IsNull.The two methods can be very different when the data volume performance is not very large, but when the data is large and the sub-query nesting complex may need to car