sql queries for practice with answers

SQL is a very basic and important knowledge in a relational database, although the backend Development class library like Laravel provides an ORM abstract data class that encapsulates a subset of simple SQL queries, so many times we don't need to relate the specifics of SQL can develop their own backend applications ve

method is recommended, mainly low costThere are several advantages:1, compared with the physical image of such technology, this scheme is more economical. Backup service hardware requirements are not high, as long as the hard disk is large enough.2. Although SQL Server provides several backup verification mechanisms, the only way to ensure full reliability of backup is to actually recover it.3, early recovery backup, so that when a real disaster occu

Name,age from-employee WHERE Age > (SELECT AVG. From employee)#查询大于部门内平均年龄的员工名, age#SELECT Dep_id,avg (age) age from employee GROUP by dep_id#SELECT d.* from Department D left joins employee E on d.id=e.dep_id#LEFT JOIN (SELECT dep_id,avg (age) age from employee GROUP by dep_id) A on d.id=a.dep_id#WHERE D.age>a.age#SELECT employee.name,employee.age from Employee INNER JOIN# (SELECT Dep_id,avg (age) age from employee GROUP by dep_id) A on employee.dep_id=a.dep_id#WHERE Employee.age>a.age#departm

Tags: greater than ADA sub-query OCA rom alt ash he not• Write multi-column subqueries• Using subqueries in the FROM clause• Use single-column subqueries in SQL• Write related sub-queries• Use the EXISTS and not EXISTS operators• Updating and deleting data using subqueries• Using the WITH clause --Multi-column subquery (unpaired comparison paired comparison)1. Check the employee_id of other employees with

sql| Statistics Writing associative queries and statistics based on several tables is really cumbersome, because many of the information based on character descriptions is difficult to extract and share, and must be rewritten when this situation is needed elsewhere. To avoid this situation, SQL artisan references the View object and describes the information of t

tables in Proc. However, you should be aware of the limitations of the proc temporary table cache.About table variables.Table variables have the same caching mechanism as temporary tables (note: table-valued parameters do not support caching)Table variables cannot be indexed, but can have a default constraintTable variable does not have statistical informationTable variable does not support transactionsAbout the explicit drop temporary table in Proc.Microsoft claims that the explicit drop tempo

T_class c on s.classid = c.classidThe result is:Summary: The right connection is exactly the opposite of left join, showing all rows of the right table, and the same row as the left table and the right table.3. Full connection:Concept: Returns all rows in the left and right tables. When a row does not have a matching row in another table, the columns in the other table return a null valueSELECT * from t_student s full join T_class c on s.classid = c.classid The result is:Summary: Returns all

* from score where degree in (85,86,88)--6, Query student table ' 95031 ' class or sex for ' female ' student records. SELECT * FROM student where class= ' 95031 ' or ssex= ' female '--7, querying all records of student table in descending class. SELECT * FROM student order by class Desc--8, CNO Ascending, Degree descending all records of the score table. SELECT * FROM Score ORDER by Cno,degree Desc--9, check the number of students in ' 95031 ' class. Select COUNT (*) ' number ' from student wh

operation. This indicates, the ' view ' would be selected from directly as opposed to being broken-to-joins on the base table S. A number of constructs make A view non mergeable. Inline views is also non mergeable. 4.partition View --Partitioned view Partition views is a legacy technology that were superceded by the partitioning option. This section of the article are provided as reference for such legacy systems. Attached: Oracle Optimizer (Optimizer) The Oracle Database Optimizer (Optimize

new empty database testusing the command: mysql -u root #因为在上一步已经退出了MySQL，现在需要重新登录 CREATE DATABASE test; #新建一个名为test的数据库 Again Ctrl + Z exit MySQL, then enter the statement to restore, the bak.sql just backed up to the test database: test We enter a command to view the table of the Tset database to verify the success of the recovery: mysql -u root #因为在上一步已经退出了MySQL，现在需要重新登录 use test #连接数据库test SHOW TABLES; #查看test数据库的表 You can see 4 tables and a