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A structure that sorts the values of one or more columns in a database table , Use indexes to quickly access specific information in database tables .

Advantages and disadvantages ： advantage ： Using index can speed up the retrieval of data , Index can be unique , Creating an index allows you to specify a single column or multiple columns

shortcoming ： Slow down the data entry speed , It also increases the size of the database

（5）order by and group by The difference between ？

order by Query for sorting 、

ASC Ascending DESC Descending

group by Query for groups 、

having It can only be used for group by Clause 、 Act on the group 、

having The query statement of condition clause needs to use aggregate function

（6）sql What are the connection methods of the tables in ？

Internal connection 、 External connection 、 Cross connect

Internal connection ：inner join in , A combination of two tables

External connection ： Divided into left connection 、 The right connection 、 Full connection

Left connection A left(outer) join B

With A Table based ,A All the data in the table ,B There are combinations of watches , What is not is null

The right connection A right(outer) join B

With B Table based ,B All the data in the table ,A There are combinations of watches , What is not is null

Full connection A full(outer) join

The two watches are the same ,A Table has ,B Data not in the table ( Is shown as null), Again B Table has ,A What the table doesn't show is null

Cross connect ：cross join, Even if Descartes product

（7） Query statements in the database is slow , How to optimize ？

① Index

② Reduce the association between tables

③ Optimize sql sentence , Try to make sql Quickly locate the data , Don't let sql Do a full table query , We should take the index , Put the large amount of data in the front

④ Simplify query fields , Useless fields do not

⑤ As far as possible with PreparedStatement To query , Do not use Statement

（8） What is a cursor ？

A cursor is a mechanism that can extract one record at a time from a result that includes multiple data records for processing .

How to use the cursor ：① Define cursors declare cursor Cursor name for select Query statement [for {readonly|update}]

② Open cursor open cursor

③ Manipulating data from cursors fetch... current of cursor

④ Close cursor close cursor

（9） How to delete the main table record , Delete the records associated from the table ？

If there is a primary foreign key relationship between the two tables , When deleting the records of the primary key table , If there is an associated record from the table, the deletion will fail

When defining foreign key constraints , You can specify at the same time 3 Delete strategies ： One is to delete the record from the table ( cascading deletion );

Second, set the foreign key field of the slave table record to NULL;

（10） List several common aggregate functions ？

sum function avg function max function min function count function

（11）oracle Basic data type ？

① String type char 、nchar 、 varchar 、 vachar2

② Numeric type number 、 integer

③ Floating point type float

④ The date type date 、 timestamp

⑤ LOB type blob 、 clob 、 nclob 、 bfile

（12）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 In paging query ,mysql use limit ;oracle use rownum

（13）union and union all What's the difference ？

union Filter duplicate records after table join , So after the table is joined, the resulting result set will be sorted , Delete duplicate records and return results

union all Just simply merge the two results and return

In terms of efficiency ,union all Than union Much faster , therefore , If you can confirm that the two result sets of the merge do not include duplicate data , So use union all

（14）truncate and delete The difference between ？

The same thing ： Both delete all rows in the table

Different places ：① truncate table Than delete Fast

② delete Delete one line at a time , And record one entry for each Deleted Row in the transaction log ;truncate Delete data by releasing the data page used to store the data , And only record the release of the page in the transaction log .

③ truncate table Delete content , Free space without deleting definition

④ delete table Delete content , Don't delete definition but don't free space

⑤ drop table Delete content and definition , Release space

（15） Transfer line column 、 How to turn a column into a line ？

① Use decode function

② Use case when sentence

（16）oracle How to go heavy ？ How to get system time ？

duplicate removal ： Use distinct keyword select distinct name from A

Get system time ：select to_char(sysdate, 'yyyy-MM-dd HH24:mi:ss') from dual;

（17） The role of sequence ？

oracle Use sequences to generate unique numbers , It is used to process an auto increment field in a table .

Once you access a serial number ,oracle Will automatically increment the next number before processing the next request , This ensures that there are no duplicate values .

（18） What is stored procedure ？ Advantages and disadvantages of stored procedures ？

Stored procedure a precompiled 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 .

Advantages and disadvantages of stored procedures ：

advantage ：① Stored procedures are precompiled , High execution efficiency

② Stored procedure code is stored 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

（19） The difference between stored procedures and functions ？

① Function has return value , The stored procedure did not return a value

② Because the stored procedure does not return a value , So we can't assign the execution result of stored procedure to variable ; Function has return value type , When you call a function , You can assign the execution result of a function to a variable .

That is to say , The function can be in select Use in statement , Stored procedures can't .

Here are some sql Sentence practice ：

-- Check the math scores of all the students

select s.name,g.score

from student s, grade g

where s.id=g.id and g.kemu=' mathematics ';

-- Count the total score of each student , Show fields ： full name 、 Total score

select a.name sum(b.score) as sum_score

from student a, grade b

where a.id = b.id group by name;

-- List the best students in each course , Request fields ： Student number 、 full name 、 subject 、 achievement

select a.id, a.name, g.kemu, max(g.score)

from student a, grade g

where a.id = g.id group by g.kemu

-- List the top three students in math

select a.id, a.name, g.kemu, g.score

from student a, grade g

where g.score = ' mathematics ' order by score limit 3;

-- Statistics English class is less than 80 Divided people

select a.id, a.name, g.kemu, g.score

from student a, grade g where a.id g.id

and g.kemu = ' English ' orader by g.score <80;

-- Check the top two in each subject

select a.id , a.name, g.kemu, g.score

from student a, grade g

where a.id = g.id

order by g.kemu limit 2;

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