Database interview

1、 Function of trigger ？

trigger Is a special stored procedure , It is mainly triggered by events . It can strengthen constraints , To maintain data integrity and consistency , You can track operations within the database so that unauthorized updates and changes are not allowed . Can be concatenated . Such as , Triggers on one table contain data operations on another , This operation will cause the table trigger to be triggered .

2、 What is stored procedure ？ What to call ？

A stored procedure is a precompile Of SQL sentence , The advantage is to allow modular design , That is to say, you only need to create , Later in the program can be called many times . If an operation needs to be performed more than once SQL, Using stored procedures is better than just SQL Statement execution should be fast . call ： 1） You can call a stored procedure with a command object . 2） It can be called by external programs , such as ：java Program .

3、 Advantages and disadvantages of stored procedures ？

advantage ： 1） Stored procedures are precompiled , High execution efficiency . 2） The stored procedure code is stored directly in the database , Call... Directly through the stored procedure name , Reduce network communication . 3） High safety , The execution of a stored procedure requires a user with certain permissions . 4） Stored procedures can be reused , Can reduce the workload of database developers . shortcoming ： Poor portability

4、 The difference between stored procedures and functions

stored procedure

1、 Used to complete specific operations or tasks in the database （ If inserted 、 Delete etc. ）

2、 The program header statement uses procedure

3、 There is no need to describe the return type when declaring the program header. You can use in/out/in out The parameters of the three modes

4、 As an independent PL/SQL sentence To execute

5、 Can pass out/in out Returns zero or more values

6、SQL sentence (DML or SELECT) Cannot call stored procedures in

function

1、 For specific data （ If you choose ）

2、 The program header statement uses function

3、 The return type should be described when the program header is declared , and PL/SQL The block must contain at least one valid return sentence

4、 have access to in/out/in out The parameters of the three modes

5、 Can't execute... Independently , Must be called as part of the expression

6、 adopt return Statement returns a value , And the change value should be consistent with the declaration part , Or through out The variable brought out by the parameter of type

7、SQL sentence (DML or SELECT) You can call functions in

5、 The function of index ？ And its advantages and disadvantages ？

Index is a special query table , Database search can use it to speed up the retrieval of data . It's very similar to the catalogue of books in real life , You don't need to query the whole book to find the data you want . The index can be unique , Creating an index allows you to specify a single column or multiple columns . The disadvantage is that it slows down the speed of data entry , It also increases the size of the database .

6、 What fields are suitable for indexing

only 、 Not empty 、 Frequently queried fields

7、 What are the index types ？

logically ：

Single column Single line index

Concatenated Multiline index

Unique unique index

NonUnique Non unique index

Function-based Function index

Domain Domain index Physically ：

Partitioned Partition index

NonPartitioned Non partitioned index

B-tree ：

Normal Normal type B Trees

Rever Key Inversion type B Trees Bitmap Bitmap index

8、 What is business ？ What is a lock ？

Transactions are bound together as a logical unit of work SQL Statement grouping , If any statement operation fails, the whole operation will fail , Later, the operation will roll back to the pre operation state , Or there's a node on it . In order to make sure that or carry out , Or not , You can use transactions . Consider having groups of statements as transactions , It needs to pass ACID test , Atomicity , Uniformity , Isolation and persistence . lock ： In so DBMS in , Lock is the key to transaction , Locks guarantee the integrity and concurrency of transactions . Like locks in real life , It can make the owner of some data , Certain data or data structures cannot be used for a certain period of time . Of course, there are levels of locks .

9、 What is view ？ What is a cursor ？

View ： It's a virtual table , It has the same function as the physical table . The view can be added , Change , check , operation , An attempt is usually a subset of rows or columns that have one or more tables . Changes to the view affect the base table . It makes it easier for us to get data , Compared to multi table query . The cursor ： It is to effectively process the result set as a unit . The cursor can be positioned on a specific line in the cell , Retrieve one or more rows... From the current row in the result set . You can modify the current row of the result set . Generally, cursors are not used , But when it comes to processing data item by item , Cursors are very important .

10、 Advantages and disadvantages of views

advantage ： 1） Access to database , Because the view can selectively select a part of the database . 2 ） Users can get results from complex queries through simple queries . 3 ） Maintain data independence , Trying to retrieve data from multiple tables . 4 ） Different views can be generated for the same data . shortcoming ： performance ： When querying the view , You have to convert a query from a view to a query on a base table , If the view is defined by a complex multi table query , that , Then you can't change the data

11、 List several ways to connect tables , What's the difference? ？

Internal connection 、 Self join 、 External connection （ Left 、 Right 、 whole ）、 Cross connect Internal connection ： Only two element tables that match can be displayed in the result set . External connection ： The left outer join : On the left is the driver table , All the data in the driver table are displayed , Mismatches in the matching table will not be displayed . Right connection : On the right is the driver table , All the data in the driver table are displayed , Mismatches in the matching table will not be displayed . Full outer join ： All mismatched data in the linked table will be displayed . Cross connect ： The Cartesian effect , The result is the product of the number of linked tables .

12、 The difference between primary key and foreign key ？

The primary key is unique in this table 、 Don't be empty , Foreign keys can be repeated and empty ; The foreign key is associated with the primary key of another table , Cannot create a foreign key that does not exist in the corresponding table .

13、 Query statements in the database is slow , How to optimize ？

1. Index building 2. Reduce the association between tables 3. Optimize sql, Try to make sql Quickly locate the data , Don't let sql Do a full table query , We should take the index , The data Big watches are at the front 4. Simplify query fields , Useless fields do not , Has control over the return result , Try to return a small amount of data 5. As far as possible with PreparedStatement To query , Do not use Statement

14、 What are the three paradigms of database ?

First normal form ： Columns cannot be subdivided Second normal form ： Rows can be uniquely distinguished , Primary key constraint Third normal form ： The non primary attribute of a table cannot depend on the non primary attribute of other tables Foreign key constraints And the three paradigms are level by level dependent , The second paradigm is based on the first paradigm , The third paradigm builds on the first and second paradigms

15、union and union all What's the difference ?

UNION Duplicate records will be filtered out after table linking , Therefore, after the table is linked, the generated result set will be sorted , Delete duplicate records and return results . In practice, most applications will not produce duplicate records , The most common are process tables and history tables UNION. UNION ALL Just simply merge the two results and return . such , If there are duplicate data in two returned result sets , Then the returned result set will contain duplicate data . In terms of efficiency ,UNION ALL than UNION Much faster , therefore , If you can confirm that the two merged result sets do not contain duplicate data , So use UNION ALL.

16、Varchar2 and varchar What's the difference? ？

Char The length is fixed , and varchar2 The length of is changeable , such as , Store string “abc” about char(20), Indicates that the characters you store will occupy 20 Bytes , contain 17 Empty , And the same varchar2（20） Only occupied 3 Bytes ,20 It's just the maximum , When you store characters less than 20 when , Store in actual length . char The efficiency of is to be varchar2 The high efficiency . at present varchar yes varchar2 A synonym for , Industrial standard varchar Type can store empty strings , however oracle You can't do that , Although it reserves the right to do so later .Oracle I developed a data type myself varchar2, This type is not a standard varchar, He will be in the database varchar Column can store an empty string property to store null value , If you want to have backward compatibility ,oracle It is recommended to use varchar2 instead of varchar

17、Oracle and Mysql The difference between ？

1） Library functions are different . 2）Oracle It's managed with tablespaces ,Mysql No . 3） Show all current tables 、 user 、 Change connected users 、 Show current connected users 、 Different statements for executing external scripts . 4） Paging query time ,mysql use limit oracle use rownum

mysql> SELECT * FROM table LIMIT 5,10; // Retrieve record lines 6-15 

// If only one parameter is given , It means to return the maximum number of record lines ： mysql> SELECT * FROM table LIMIT 5; // Before retrieval 5 A record line  

// To retrieve all record lines from an offset to the end of the recordset , The second parameter can be specified as -1： mysql> SELECT * FROM table LIMIT 95,-1; // Retrieve record lines 96-last.