MySQL learning records -- III. MySQL query statements

One 、mysql data type

1. value type
1.1.MySQL Support all standards SQL Numerical data type .
1.2. These types include strict numerical data types (INTEGER、SMALLINT、DECIMAL and NUMERIC), And approximate values
1.3. According to the type (FLOAT、REAL and DOUBLE PRECISION).
1.4. keyword INT yes INTEGER A synonym for , keyword DEC yes DECIMAL A synonym for .
1.5.BIT Data type save bit field value , And support MyISAM、MEMORY、InnoDB and BDB surface .
1.6. As SQL Standard extension ,MySQL Integer types are also supported TINYINT、MEDIUMINT and BIGINT. The following table shows the storage and range required for each integer type .

type	purpose	size	Range （ A signed ）	Range （ Unsigned ）
tinyint	Small integer value	1Bytes	(-128, 127)	(0, 255)
smallint	Large integer value	2Bytes	(-32768, 32767)	(0, 65635)
mediumint	Large integer value	3Bytes	(-8 388 608, 8 388 608)	(0, 16 777 215)
int or integer	Large integer value	4Bytes	(-2 147 483 648, 2147483648)	(0, 4294967275)
bigint	Maximum integer value	1Bytes	(-128, 127)	(0, 255)
float	Single precision Floating point numbers	4Bytes	(-3402823466E+38, -1.175494351E-38) ,0,(1.175494351E-38,3.402823466351E+38)	(0, (1.075494351E-38,3.402823466E+38))
double	Double precision Floating point numbers	8Bytes	(-1.7976931348623157E+308, -2.2250738585072014E-308),0,(2.2250738585072014E-308),0,(2.2250738585072014E-308,1.797693134868315E+308)	(0, 255)
decimal	Small value	Yes DECIMAL, If M>D, by M+2 otherwise D+2	Depend on M and D	Depend on M and D

2. Date and time type
2.1. The date and time type that represent the time value are DATETIME、DATE、TIMESTAMP、TIME and YEAR.
2.2. Each time type has a valid value range and a " zero " value , When the designation is illegal MySQL Use... When the value cannot be represented " zero " value .
2.3.TIMESTAMP Type has a proprietary automatic update feature , Will be described later .

type	purpose	size	Range	Format
date	Date value	3Bytes	1000-01-01/9999-12-31	YYYY-MM-DD
time	Time value or duration	3Bytes	‘-838:5959’/‘838:59:59’	HH:MM:SS
year	The year is worth	1Bytes	1901/2155	YYYY
datetime	Mix date and time values	8Bytes	1000-01-01 00:00:00/9999-12-31 23:59:59	YYYY-MM-DD HH;MM;SS
timestamp	Mix date and time values , Time stamp	4Bytes	1970-01-01 00:00:00/2038	YYYYMMDD HHMMSS

3. String type
String type means CHAR、VARCHAR、BINARY、VARBINARY、BLOB、TEXT、ENUM and SET. This section describes how these types work and how to use them in queries .

type	purpose	size
char	Fixed length string	0-255 Bytes
varchar	Variable length string	0-65535 Bytes
tinyblob	No more than 255 Binary string of characters	0-255 Bytes
tinytext	Text string	0-255 Bytes
blob	Long text data in binary form	0-65535 Bytes
text	Long text data	0-65353 Bytes
mediumblob	Medium length text data in binary form	0-16777215 Bytes
mediumtext	Medium length text data	0-16777215 Bytes
longblob	Maximum text data in binary form	0-4294967295 Bytes
longtext	Large text data	0-4294967295 Bytes

Two 、 mysql Query operation

1. You can use one or more tables , Use commas between tables , Division , And use WHERE Statement to set the query conditions .
2. Use AND perhaps OR Specify one or more conditions .
3.WHERE Clause can also be applied to SQL Of DELETE perhaps UPDATE command .
4.WHERE Clause is similar to... In programming languages if Conditions , according to MySQL Table to read the specified data .

Search method

select * from surface where Conditions

// Search for 
SELECT * FROM `user` WHERE real_name = ''
// Fuzzy search 
SELECT * FROM `user` WHERE real_name LIKE '% Yang %'
// Multiple requirements 
SELECT * FROM  Table name  WHERE  Parameters  = '' AND  Parameters  = ''
// Select a table to query 
SELECT * FROM  Library name . Table name  WHERE  Parameters  = ' anhui '

Multiple tables associated query

select Output content
from surface a Table alias （ Better be simple ）
left join surface b Table alias （ Better be simple ） on a.xx( Table parameters ) = b.xx( Table alias )
( Grouping output )group by a Table alias ,xx

// Multi-table query （ Left 、 Right 、 Internal connection （ The mapping relationship ））
SELECT
 c.xx AS '',
 count(p.xx) AS ''
FROM
  Table name  c
LEFT JOIN  Table name  p ON c.xx = p.xx
LEFT JOIN   Table name  t on p.xx= t.xx
WHERE
c.xx= 1 
and c.add_time BETWEEN '2022-04-01 00:00:00' and '2022-05-24 00:00:00'
AND p.xx = 1 and t.xx = 1
GROUP BY c.xx

details

a、 Even the table 
     No correspondence is shown 
    select A.num, A.name, B.name
    from A,B
    Where A.nid = B.nid
 
     No correspondence is shown 
    select A.num, A.name, B.name
    from A inner join B
    on A.nid = B.nid
 
    A Table all display , If B There is no correspondence in , Then the value is null
    select A.num, A.name, B.name
    from A left join B
    on A.nid = B.nid
 
    B Table all display , If B There is no correspondence in , Then the value is null
    select A.num, A.name, B.name
    from A right join B
    on A.nid = B.nid
b、 Combine 
     Combine , Automatic processing coincidence 
    select nickname
    from A
    union
    select name
    from B
 
     Combine , Unprocessed coincidence 
    select nickname
    from A
    union all
    select name
    from B

Query add parameters
as： Take the alias select x.id as ’ name ‘ where …
group by( Must be in where after ,order by Before )： grouping , There is a mapping relationship , branch （ Column ） Group shows a parameter
limit/limit X offset X： Limit , Find out the information between (select * from surface limit 5;)
between： Find out the goals between （select * from Table name where c. Time BETWEEN ‘2022-04-01 00:00:00’ and ‘2022-05-24 00:00:00’）
in： Look up a line Example ：select * from surface where id in (11,22,33)
on： Mating left chain 、 Right chain 、 Internal links use , ditto （ Multi-table query ）
like(%/_ Both front and back )： Fuzzy query （select * from surface where name like ‘ale%’）% Multiple strings , （select * from surface where name like ‘ale_’）_ Single string
Aggregate function query
COUNT： Returns the total number of records whose specified column is not empty
SUM:： Sums the specified columns ( Such as salary payment )
MIN, MAX：MIN(e1) return e1 The minimum value in the column specified by the expression ; MAX(e1) return e1 The maximum value in the column specified by the expression ;
desc/desc：(select * from surface order by Column 1 desc, Column 2 asc) - according to “ Column 1” From large to small , If they are the same, press the column 2 Sort from small to large
AVG： The column specified for the expression , averaging .
MEDIAN： First , according to e1 The column specified by the expression , Sort values ; If after sorting , The total record is odd , Return to the sorting queue , Values in the middle ; If after sorting , The total record is even , Is in the sort queue , Average the middle two values , Return this average ;
***RANK(RANK( ) OVER ([ PARTITION BY column1 ] ORDER BY column2 [ASC|DESC]))***( Find the sequence number of a parameter )：
（select rank(1500) within group (order by salary) as “rank number” from employees）
FIRST、LAST：

 grammar ：    agg_function（e1） KEEP (DENSE_RANK FIRST ORDER BY e2 [NULLS {
    FIRST|LAST}]) [OVER PARTITION BY e3 ]
            agg_function（e1） KEEP (DENSE_RANK LAST  ORDER BY e2 [NULLS {
    FIRST|LAST}]) [OVER PARTITION BY e3 ]                  
         Parameters ：    agg_function For an aggregate function , It can be for  MIN、MAX、SUM、AVG、COUNT、VARIANCE or STDDEV
            e2 Specify which field to base on , Sort ;
            e3 Specify which field to base on , To classify （ grouping ）;
             When specifying OVER PARTITION BY After clause , Sort each class separately after classification ;
            DENSE_RANK Assign sequence numbers to the sorted records , And the serial number is continuous .
            NULLS {
    FIRST|LAST} Specify sort fields e1 If the value of is empty , Then shoot in front of the sequence （NULLS FIRST） Or back （NULLS LAST）
            DENSE_RANK After FIRST/LAST Confirm selection passed DENSE_RANK In a sequenced sequence , The serial number is the smallest / The biggest record . When the serial number is the same , Return multiple records 
             When the serial numbers are the same , When multiple records are returned ,agg_function(e1) The aggregate function continues to process the multiple records e1 Fields are aggregated .
         effect ：     If agg_function by min(e1), Get the sorted FIRST or LAST In multiple records of , A field e1 The minimum value of 
             This field is not a sort key field e2
         example ：
         The employee table is known to have a salary field , Bonus field . Of the lowest paid employees , The record of the employee with the highest bonus .
         The known table is as follows ：
        SQL> select * from employees order by salary;
         EMP_ID     EMP_NAME           SALARY  COMMISSION
        ---------- ---------------------------- ------------  ------------
        10001      ZhangSan              500        200
        10002      LiSi                  500        300
        10003      WangWu                500        100
        10004      MaLiu                 2000       500
        10005      NiuQi                 2500       200
        10006      ShangDuo              2500       300
        10007      BaiQi                 2500       400
        
        SQL> select max(commission) keep(dense_rank first order by salary asc) as commission from employees;
        COMMISSION
        ----------
               300
         First , Press salary After ordering , Get a record of the lowest salary , They are employees 10001、10002、10003 Three records .
         Aggregate functions max(commission) Yes 3 The employee who gets the highest bonus in the records 10002, Bonus is 300.

3、 ... and 、 mysql The insert

MySQL Used in table INSERT INTO SQL Statement to insert data .
Insert method

INSERT INTO user (user_name,user_pwd,user_phone,user_time) VALUES (?, ?, ?, ?)

INSERT INTO `user` (user_name,user_pwd,user_phone,user_time) VALUES (?, ?, ?, ?)

Four 、 mysql update operation

update surface set name ＝ ‘alex’ where id>1

update  surface  set name ＝ 'alex' where id>1

5、 ... and 、 mysql Delete operation

delete from surface where id＝1 and name＝’alex’

delete from  surface 
delete from  surface  where id＝1 and name＝'alex'

// Welcome to add