sql study guide

queries the second field and finds that the index is not working. 12. Do not write meaningless queries, such as the need to generate an empty table structure: (General development will not be so boring, on the official project to write this kind of thing)Select Col1,col2 into #t from T where1=0 This type of code does not return any result sets, but consumes system resources and should be changed to this:CREATE TABLE #t (...)Reference:https://mp.weixin.qq.com/s?__biz=MzIxMjg4NDU1NA==mid=22474836

[Database Study Notes] (3) ing between SQL data types and Java data types, learning notes Data Types Comparison of Data Types Between SQL database and access Database Text nvarchar (n)Note ntextNumber (long integer) intInteger smallintNumber (single precision) realNumber (Double Precision) floatNumber (byte) tinyintCurrency moneyDate smalldatetimeBooleanIng be

empname,b.ename mgrnameFrom EMP A,emp bwhere A.mgr=b.empno;2. List the names of employees who work in the "IT" department;Select ename from empwhere deptno=(Select Deptno from deptwhere dname= ' IT ');3. Name, number and department number of all employees whose wages are higher than the average wage of the company;Select Ename,deptno,empno from empWhere sal> (select AVG (SAL) from EMP);4, list the number of employees in each department, the average salary;Select AVG (SAL), COUNT (*)From EMPGrou

that the tuple is identical. since Studentnum and Lessonnum are federated primary keys, it is not possible to filter with distinct because each primary key (in this case, the Federated primary key) is different, i.e. the actual primary key is (A, b) (A,c) (b,c) (b,d) In this form, they are different, and naturally you cannot use distinct to filter the duplicate values contained in one of the values. (7) Violent replication of table data, INSERT into statement is very useful, and for the first t

There are three types of data changes in the database: insert, update, and delete ). These operations are common to developers. There are three types of data changes in the database: insert, update, and delete ). These operations are common to developers. There are three main types of data changes in the database: insert data ( InsertOperations and precautions UpdateOperations and precautions DeleteOperations and precautions Through the study in

There are three main types of data changes in the database: insert data (Insert), Modify data (Update), Delete data (Delete). These operations are commonly used by developers. This chapter briefly introduces these operations. The main contents of this chapter include: InsertOperations and precautions UpdateOperations and precautions DeleteOperations and precautions Through the study in this chapter, readers can learnInsert/update/deleteThe basic opera

changes are written to the database.When and whether you really need to call Submitall () actually depends on the table's edit policy (edit strategy), the default policy is Qsqltablemodel::onrowchange, This means that when the user selects a different row, the changes to the row are applied to the database. Other strategies include qsqltablemodel::onmanualsubmit(all changes will be cached in the model until you call the Submitall () method) and Qsqltablemodel::o Nfieldchange (no changes are cac

DC--perform connection, most recently executed query informationCross apply sys.dm_exec_sql_text (dc.most_recent_sql_handle) DTJoin Sys.dm_exec_sessions DS on dc.session_id=ds.session_idwhere Ds.login_name= ' LCGS609999 '--where ds.program_name = '. Net SqlClient Data Provider 'ORDER by Dt.textCopy Code Copy Code--Clears all cached data in the buffer poolDBCC dropcleanbuffersGO--Clears all cached execution plans in the buffer poolDBCC FreeproccacheGO--statistical information, implementation pla

Oracle Study--pl/sql Developer software errorThe blog is open at 51cto and CSDN.System environment:Operating system: Windows XP (64)Software: PL/SQL Devoloper 8.04Software, after normal installation, the following error occurred during use:650) this.width=650; "Src=" http://img.blog.csdn.net/20150408162908002?watermark/2/text/ ahr0cdovl2jsb2cuy3nkbi5uzxqvbhf4mdqw

SQL Study Notes: stored procedures and user-defined functions I. Simple creation, modification, and deletion of Stored Procedures1. Create a simple Stored ProcedureUse adventureworksGoCreate proc spemployeeAsSelect * From HumanResources. run the code above to create a stored procedure. If you want to run this stored procedure, you can directly execute the exec spemployee Statement 2. alter proc [DB

Oracle Database pl SQL study notes, oraclepl 1. define basic variables:2. referenced variable: set serveroutput on declarepename emp. ename % type; psal emp. sal % type; beginselect ename, sal into pename, psal from emp where empno = '000000'; dbms_output.put_line (pename | 'salary:' | psal); end; /3. Record-type variables: set serveroutput on declareemp_rec emp % rowtype; beginselect * into emp_rec from em