find duplicates query sql

I. Operator Optimization 1. In Operator SQL statements written in are easy to write and understand, which is suitable for modern software development. However, SQL statements using in always have low performance. The following differences exist between SQL statements using in and SQL statements without in: Oracle

In the database query, there is a complete query and fuzzy query points.General Fuzzy query statements are as follows: SELECT field from table WHERE a field like condition With regard to the conditions, SQL provides four matching modes:1,%: Represe

We have to do not only write SQL, but also to write good performance of SQL statements.(1) Select the most efficient table name order (valid only in the rule-based optimizer):The Oracle parser processes the table names in the FROM clause in a right-to-left order, and the FROM clause is written in the final table (the underlying table, driving tables) will be processed first, and in the case where the FROM c

(SELECT DISTINCT job from EMP where deptno=10), query table with the same staff with department Number 10 The name, work, salary, department number of the worker. Because there are multiple rows in the returned result, the subquery is concatenated with "in".In and exists: exists () The subquery that follows is called the correlated subquery, which is not the value of the returned list. Just returns the result of a ture or false, which runs the main

related subqueries, you can use the EXISTS and not exists operators to determine whether a value is in a series of values. When SQL Server Processes subqueries with the EXISTS and NOT EXISTS operators: Whether the record returned by the outer query test subquery exists Based on the conditions specified by the query, the subquery returns TRUE or FALSE Subqueries

the directory just now, here is a sample PHP program:$display _order = Array ( 1 = 4, 2 = 1, 3 = 2, 4 = 3, 5 = 9, 6 = 5, 7 = Gt 8, 8 = 9); $ids = Implode (', ', Array_keys ($display _order)); $sql = "UPDATE categories SET display_order = case ID"; foreach ($display _order as $id = + $ordinal) { $sql. = sprintf ("When%d and%d", $id, $ordinal); Splicing

For a slow query, we usually need to know which steps are expensive first, and then try some improvement methods. In general, if you cannot solve the problem by improving the hardware performance or adjusting the OS or SQL server settings, the remaining options are usually the following:1. Add the index of the corresponding field for the "scan" operation.2. Sometimes re-indexing may also be effective. For d

', ' Maliniak ', ' M ', ' 1989-09-12 '); INSERT into Employees VALUES (10006, ' 1953-04-20 ', ' Anneke ', ' preusig ', ' F ', ' 1989-06-02 '); INSERT into Employees VALUES (10007, ' 1957-05-23 ', ' Tzvetan ', ' Zielinski ', ' F ', ' 1989-02-10 '); INSERT into Employees VALUES (10008, ' 1958-02-19 ', ' Saniya ', ' Kalloufi ', ' M ', ' 1994-09-15 '); INSERT into Employees VALUES (10009, ' 1952-04-19 ', ' sumant ', ' peac ', ' F ', ' 1985-02-18 '); INSERT into Employees VALUES (10010, ' 1963-

Tags: PID pool table mina ext execution SID proc App--Query for the slowest SQL SELECT * FROM (Select Parsing_user_id,executions,sortsCommand_type,disk_reads,sql_text from V$sqlarea ORDER BY disk_reads Desc) Where rownum --Query corresponding session Select SE. Sid,se. Serial#,pr. SPID,TEI Username,se. Status,se. TERMINAL,TEI Program,se. MODULE,TEI Sql_address,st

, "language"),(2, "mathematics"),(3, "English");Query OK, 3 Rows Affected (0.00 sec)Records:3 duplicates:0 warnings:0 (Note: The above actions can also be executed with the SQL statement set: (The advantage is that the other machine can also execute the SQL statement set on the line, do not have to play from scrat

highly efficient, but because of its limited statistics, the optimizer's work process is more likely to deviate. For complex multiple table joins, on the one hand because of its optimizer limited, and also in the Join this aspect of the effort is not enough, so performance from Oracle and other relational database predecessors still have a certain distance. However, if it is a simple single table query, this gap will be very small even in some scenar

Generate serial number during QuerySelect (select count ([xlh]. [AA]) as autonum from xlh where (xlh. aa) From xlh as xlh_alias inner join xlh on xlh_alias.aa = xlh. AAOrder by xlh. AA; Multi-Table SQL querySelect Test. AA as the first field, test1.bb as the second field, test1.ccFrom test, test1Where test. AA = test1.aa; Multi-Table SQL query 1Select a. AA, B.

, so you don't need to scan the table to quickly find the data you need.the role of database cursors ( cursor ):A cursor is actually a mechanism that extracts one record at a time from a result set that includes multiple data records. Cursors act as pointers. Although the cursor can traverse all the rows in the result, he points to only one row at a time.In summary, a SQL cursor is a temporary database obje

Introduction The view can be viewed as a virtual table defined on SQL Server. A view, like its name, is another entry to view data. the conventional view itself does not store actual data, but only stores a SELECT statement and metadata of the involved table. A simple understanding of the view is as follows: Through the view, the client no longer needs to know the structure of the underlying table and Its Relationship. The view provides an

Document directory Http://www.cnblogs.com/CareySon/archive/2011/12/07/2279522.html Introduction The view can be viewed as a virtual table defined on SQL Server. A view, like its name, is another entry to view data. the conventional view itself does not store actual data, but only stores a Select statement and metadata of the involved table. A simple understanding of the view is as follows: Through the view, the client no longer needs to kno