sql nested query example

The SELECT clause is logically the last step in the final processing of the SQL statement, so the following query will have an error: SELECT year (OrderDate asorderyearCOUNT(DISTINCT CustomerID fromdboGROUP by OrderYear; Because group by was preceded by a SELECT, the OrderYear column was not formed at that time. If you want to query for su

T-SQL query advanced-understanding the lock introduction in SQL Server, each query will find the shortest path to achieve their goals. If the database only accepts one connection, only one query is executed at a time. Therefore, queries must be completed quickly and easily.

(1) retrieve all rows and columns from the table Problem View All data in a table. Solution Use the SELECT statement for the table and the special character "*". select * from emp Discussion The "*" symbol in SQL has special significance. You can use this function to return each column in a specified table. Because there is no WHERE clause, each row in the table is returned. Another alternative is to list each column separately. select empno,ename,job

correctly and merge the database tables reasonably (although sometimes it may affect the normalization of the table, but it is worthwhile to improve the efficiency). If sorting is unavoidable, you should try to simplify it, such as narrowing the range of sorted columns. 3. Eliminate sequential access to large table row data In nested queries, sequential access to a table can have a fatal effect on query ef

execution plan? How can we get the optimal execution plan by writing our SQL statements? 1. Column Query Optimization If you use the column in the table, select * is not required. Unless you use each column, because select * is the same as all select columns. 2. Optimization of where query Conditions Do not perform operations on condition columns. For

The SQL statement used to query multiple fields, query multiple tables, and delete duplicate records. SQL repeat record Query 1. Search for redundant duplicate records in the Table. duplicate records are determined based on a single field (peopleId ). select * from people wh

Operations):bitmap operators in parallel processing and execution plans in SQL Server 2000We can see the execution tree using SET STATISTICS profile, here are some of the names of the columns, not to mention in more detail, there is a lot of information on the InternetNodeId: Tree nodeParent: Father NodePhysicalop: physical operatorLogicalOp: Logical operatorsThe execution model of this execution tree is not only applied to

Oracle The paging query statement of Oracle can basically follow this article. The next article will explain it through examples. Next we will briefly discuss the situation of Multi-table join. For join queries of the most common equivalent tables, CBO may use two join Methods: nested loop and hash join (merge join is less efficient than hash join, which is not considered by CBO in general ). Because paging

Label:Recently encountered a more interesting problem in the project, the online search for some methods, summarized here to share. We often encounter such a scenario: the need to query data, there are some query conditions, but when the query, we hope that when a condition is empty, then do not filter this condition, the main idea is to deal with the following:

smaller tables. The complexity of Merge joins (merge join) is O (M + N), but the connection is heavily dependent on the index on the connection column and, without an index, the rows are sorted based on the key used in the connection: If the two tables are sorted based on the key used in the connection, the query has a complexity of O (M + N). If all two tables have indexes on the join column, the indexes maintain the

topic that is beyond the scope of this article.The query optimizer chooses the most efficient connection type to connect to the data table. For example, the outer table of a nested loop join should be the smaller of the two tables connected. The hash join is the same, and its external table should be the smaller table. If you think the

apply to non-recursive CTE. For guidelines for recursive CTE, see "Guidelines for defining and using recursive CTE" later in this article. The CTE must follow a single SELECT, INSERT, UPDATE, or DELETE statement that references some or all of the CTE columns. You can also designate a CTE as part of a SELECT definition statement in a view in the CREATE view statement. Multiple CTE query definitions can be defined in a non-recursive CTE. T

SQL query method essence set, SQL method essence set 1. Simple Query A simple Transact-SQL query only includes the select list, FROM clause, and WHERE clause. They indicate the queried columns, the queried tables or views, and the

: Reduce the size of the index Indexes are reduced, resulting in faster queries to the index Small index for adding and removing changes in the maintenance performance will be higher For example, the following statement: After we have set up a clustered index for it: At this point we add filter conditions to form a filtered index: From the above we can see that the use of filtered index of the scene to be related to the spec

;SELECT sname as name, Sage age from student;SELECT sname,sage+1 sage from student;SELECT sname sage from student; --there is no problem with the grammar, it is problematic from the application level. The student's name was queried, but the result was that the column name was changed to Sage.6) NestingIn an SQL statement, a query statement can be used in a nested

of Merge joins (merge join) is O (M + N), but the connection is heavily dependent on the index on the connection column and, without an index, the rows are sorted based on the key used in the connection: If the two tables are sorted based on the key used in the connection, the query has a complexity of O (M + N). If all two tables have indexes on the join column, the indexes maintain the columns sequentially, and do n

[Name],subject,score Here for a good understanding only one column to get the following results With the example of the language accomplishment line train, it is easy to add the other two columns, Select [Name],Chinese =caseWhen subject= ' language ' then score else 0EndMathematics =caseWhen subject= ' math ' then score else 0EndForeign Language =caseWhen subject= ' foreign language ' then score else 0EndFrom StudentscoreGroup BY [Name],subject,scor

Label:The first is the problem of fuzzy query, I started with the following conditions: SELECT * from the user where name like '% #value #% '. But how can not, as if also reported wrong. Later on the internet found a solution, is to use $ to replace the # number.1> write: Like '% $value $% ' can be,2> also found another method, but that method I tried for a long time, is not, the method is: like '% ' | | #value # | | '% ', the

new record. There are three Connection Methods: inner connection, outer connection, and cross connection. In a SELECT statement, multiple tables can be connected. by extending the FROM clause of the SELECT statement, two keywords are added: JOIN and ON. JOIN: Specifies the table to be linked. ON: Specifies the fields shared by these tables. Specify the connection conditions based on the primary key and external key of the table. The ANSI link syntax is as follows: SELECT table_name.column_name,

emp WHERE deptno = 30); 6. subqueries related to subqueries in the from clause and subqueries are subqueries that need to reference the primary query table column, is implemented through the EXISTS predicate. Show all employees working in new york: SQL code 1. SELECT ename, job, sal, deptno FROM emp where exists (SELECT 1 FROM dept WHERE dept. deptno = emp. deptno AND dept. loc = 'New YORK '); When a subqu