Knowledge points of MySQL (7)

One 、 Process control functions

Process processing functions can be based on different conditions , Perform different processes , Can be in SQL Statement to implement different conditional choices .MySQL The process processing functions in mainly include IF()、IFNULL() and CASE() function .

 IF usage ：

mysql> select if(1>0,' correct ',' error '),if(1<0,' correct ',' error ') from dual;

mysql> select if(1>0,' error ',' correct '),if(1<0,' error ',' correct ') from dual;

  Two 、 Encryption and decryption functions

The encryption and decryption function is mainly used to encrypt and decrypt the data in the database , Prevent data from being stolen by others . These functions are very useful in ensuring database security .

Be careful ： stay mysql8.0 in ,password(),encode()、decode() Abandoned .

mysql>  select md5('mysql'),sha('mysql'),md5(md5('mysql')) from dual;

（ Be careful ：md5 It's irreversible ）

  3、 ... and 、MySQL Information functions

MySQL There are some built-in functions that can be queried MySQL Information functions , These functions are mainly used to help database developers or operation and maintenance personnel better maintain the database .

 mysql> select version(),connection_id(),database(),schema(),user(),current_user() from dual;

  Four 、 Other functions

MySQL Some functions in cannot classify them specifically , But these functions are MySQL In the process of development and operation and maintenance of .

about format Say when n Less than or equal to 0 when , Then only the integer part

mysql> select format(123.123,2),format(123.123,-2) from dual; 

conv：

mysql> select conv(16,10,2),conv(8888,10,16),conv(null,10,2) from dual;

inet_aton,inet_ntoa:
mysql> select inet_aton('192.168.1.100'),inet_ntoa(3232235876) from dual;

（ and 3232235876 By 192 multiply 256 Of 3 Power , add 168 multiply 256 Of 2 Power , add 1 multiply 256, Plus 100）

  5、 ... and 、 Aggregate functions

Aggregate functions （ Group function ）： Is a function that summarizes a set of data , The input is a set of data , The output is a single value .

1, Common aggregate functions

1）AVG / SUM

mysql> select avg(id),sum(id) from employees;

2）MAX / MIN（ For numeric types 、 String type 、 Field or variable of date time type ）

mysql> select max(id),min(id) from employees;

3）COUNT

(1） effect ： Calculate the number of times the specified field appears in the query structure

mysql> select count(id) from employees;

(2） Be careful ： When calculating the number of occurrences of the specified field , It is not calculated NULL It's worth it .

(3）AVG=SUM / COUNT

mysql> select avg(id),sum(id)/count(id) from employees;

2,group by Use

mysql> select id from employees group by id;

 

Be careful :

1,select The fields of non group functions appearing in must be declared in group by in , conversely ,group by The fields declared in may not appear in select in . 

2,group by The statement is in from after 、where Back 、order by front 、limit front

3,MySQL in group by Use in with rollup, You can get a new average result

mysql> select id from employees group by id with rollup;