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In reality, there is such a requirement. Sometimes the data we need to query is in multiple tables. How can we query data from multiple tables? In this case, you need to use table connections for queries.
In reality, there is such a requirement. Sometimes the data we need to query is in multiple tables. How can we query
Inner join ).External Connection:Full join, left join, and right join ).Cross join ).The outer join and the inner join are different. The query results returned by the outer join include not only the rows that meet the conditions, but also the left table (left Outer Join) and the right table (right Outer Join) or all non-conforming data rows in two connected tables (all external connections.1. left join (left [outer] join)The left Outer Join combines
No matter what database I use, duplicate data exists. Next I will introduce in detail various methods and Performance Analysis for deleting duplicate records in Oracle tables, for more information, see.
Method:
The Code is as follows:
Copy code
Group by XX having count (*)> 1, rowid, distinct, temporary table, procedure
The following statement can query the data that is duplic
Tags: bracket ROM query SQL statement from Ora reason here sqlFor example: SELECT ename, Job, Deptno from emp WHERE Deptno in (Ten) NB sp; and (ename like '%I ' or job like '%er '); The executes the SQL with the following result set: 1 jones manager NBS P 2 clark manager Remove the upper sq l parentheses in the
Use this SQL statement
select * from tree
View the raw data as follows:
The following figure shows the result of a tree query (including the fields root, level, is_leaf, and Path ):
Execute the following SQL statement:
select connect_by_root(child_col) root, level , decode(connect_by_isleaf,0,'No',1,'Yes') is_leaf, sys_connect_by_path(child_col,'/') pathfrom
------------------------1. Simple SQL query--------------------------SELECT * from EMP;Select Empno,ename,job from EMP;Select ' number is: ' | | Empno | | ' Name is: ' | | Ename from EMP;Select distinct empno, ename from EMP;Select empno,ename,job,sal*100 Income from EMP;------------------------2. Limit query --------------------------SELECT * FROM EMP where sal>
Scalar Query
Oracle allows a single-row subquery to be included in the select clause. Using scalar queries can effectively improve performance. When using external connections or using aggregate functions, we can consider the possibility of scalar quantum queries.
1. Cancel the use of external connections
External Connection practices:Select a. username, count (*) from all_users A, all_objects B
Where a. Us
In the Where clause, you can use the Like clause with wildcards to select columns of the datetime, char, and varchar field types... "Data records. The following are available wildcard characters:% Zero or multiple characters_ Any single character (underline)Special characters[] Characters in a certain range, such as [0-9] or [aeth][^] Characters out of a certain range, such as [^ 0-9] or [^ aeth]SQL provides four matching modes for conditions:1, %: re
The syntax of SQL fuzzy query is"Select column from table where column like 'pattern ';".SQL provides four matching modes:1.% indicates any 0 or multiple characters. Statement:Select * from user where name like '% 3% ';We will find all the "three" names for "Michael", "three-legged cats", and "Tang sanzang;2. _ represents any single character. Statement:Select *
match the criteria./************************** right outer connection **/select j_c.*,j_sc.* from course j_c R join SC j_sc on j_c.cno = J_sc. CNO;650) this.width=650; "src=" Http://s3.51cto.com/wyfs02/M01/49/BD/wKiom1QZkUHTlxEoAAFwxqJ2Atc312.jpg "title=" Right outer connection. png "alt=" wkiom1qzkuhtlxeoaafwxqj2atc312.jpg "/>4. Full-Outer connectionFull outer joins: Returns rows that do not match the criteria in the left and right tables, in addition to and return rows that meet the criteria/
In the case of a large record, the not in query will certainly be slow and unacceptable. Like what:SELECT A.* fromWHERE A.user_id not in (SELECT B.user_id from Table_b)the best way to find it now is to check it in seconds. For example, query the DEPT_ID data with table A and b not available:SELECT A.*from tb_dept A, Tb_dept_lev BWHERE= b.dept_id (+) and is NULLThe (+) number can be understood as: + is a
Table query:
Merge query: Use the Union keyword to remove duplicate rows that meet the criteria.
Copy Code code as follows:
Select Ename,sal,job from emp where sal > 2500 Union Select ename,sal,job from emp where job = ' MANAGER ';
The union all usage is similar to union, but does not cancel duplicate rows.
Intersect is used to take the intersection of two results.
Minus is used to
Query traversal, which needs to be stored in a hierarchical structure of data in the base table. For example, an organization is a typical example of this:
Implementation statement:
Select column
From table_name
Start with Column=value
Connect by prior Parent PRIMARY KEY = child foreign key
Example 1: In Oracle's EMP table, each record has a MGR column that uniquely identifies the current employee's empno and identifies the em
the remote query, instead of returning the result locally, the filter is executed locally (with a filter)This will certainly be very slow, because the query does not use the index lookup, you need to transfer all the remote data to the local before the filtering, equivalent to full table scan, but also more network transmission time.3. ResolveIn fact, you can resolve performance issues by committing
Label:Recently, a simple study of the data paging query of oracle,mysql,sqlserver2005 has been made, and the statements of each query are posted for everyone to learn. (a), the MySQL paging query MySQL paging query is the simplest, with the help of keyword limit can be
After reading this blog post, Oracle, MYSQL, SQL Server data paging query, in accordance with the company's Regionres table madePaging query statements for SQL Server:SELECT DISTINCT TOP -rr.id asID, Rr.strname asname, Rr.ntype asRes_type, Rr.nregid asregion_id, Rr.strchann
NUMBER
Hash value of the action that's named in the ACTION column
SERIALIZABLE_ABORTS
NUMBER
Number of times the transaction fails to serialize, producing ORA-08177 errors, totalled through all the child cursors
CPU_TIME
NUMBER
CPU time (in microseconds) used by the cursor for parsing/executing/fetching
ELAPSED_TIME
NUMBER
Elapsed time (in microseconds) used by the cursor for parsing/executing/fetching
First look at how SQL writesSelect * from ( Select A.*, ROWNUM rn from (lowest query statement ) A wherelt; = #{endcol} ) where gt; #{startcol}Note: Of course we also need select COUNT (*) the lowest query statement to get the total number of result sets. And then
) A. Need to display the account opening date under each customer account B. Need to display the date on which the customer placed the order C. Need to display all orders placed on a specific date D. Need to display all orders with the same release date as the order numbered 25950 8. Which of the following SELECT statements should be used if you want to display the product names in the report that cost values are higher than the average cost of all products? (B) A. Select Product name from produ
will look at examples. The following is a brief discussion of the multi-table syndication scenario. For the most visible equivalent table connection query, the CBO may generally use two connection modes nested loop and hash join (the MERGE join efficiency is less efficient than the hash join, and the general CBO will not consider it). Here, because paging is used, the maximum number of records to be returned is specified, and the NESTED loop can imme
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