MySQL Advanced ---- Filter conditions

select * from ... where ...;

Usually we don't need to look at all the rows of a table , What we need to look at is a line with certain conditions . front MySQL In the basic study of using , I used simple conditional filtering where age > 20 , This article will focus on where The conditions are more detailed ! I haven't learned the basic content 【MySQL Getting started 】

Simple screening

Symbol meaning
> Greater than
< Less than
= be equal to
>= Greater than or equal to
<= Less than or equal to
!= It's not equal to
<> It's not equal to

Compound condition --and 、or and not

To learn MySQL My friend, I acquiesce that you have learned at least one programming language .

A condition is a logical expression , The result of the expression is only true and false. Multiple expressions can use and and or To connect , respectively And and or , You can also use not To express Not . Here are some examples of specific uses .

--  Older than 10 And less than 20
select s_name from students where age > 10 and age < 20;
-- Age is less than 10 Or greater than 20
select s_name from students where age < 10 or age > 20;
-- Not ( Age is less than 10 Or greater than 20), Equivalent to age not less than 10 And not more than 20
select s_name from students where not (age < 10 or age > 20);
  • In the use of and and or We need to pay attention to their priority , Let's look at the following requirements
--  Search out all age groups for 20 or 21 year , Good grades (90 More than ) Information about our students 
select * from students where age = 20 or age = 21 and score >= 90;
-- The above sentence seems to be the result we want , In fact, the result is not what we want .
-- The actual meaning of the above condition is :20 I'm a 12-year-old student or 21 I'm a 20-year-old student with excellent grades
-- Why is that? , The reason is that MySQL in and The priority ratio or Higher , This leads to the combination of the latter two conditions , That is to say
select * from students where age = 20 or (age = 21 and score >= 90);
-- So in order to achieve our original intention , It should be written as follows
select * from students where (age = 20 or age = 21) and score >= 90;

When you don't know how priorities are set , Using brackets is always a safe choice , And it makes the meaning of the condition clearer . I believe you have the same experience in other languages .

String filter

= This filter is obviously not enough for Strings , Let's introduce like and regexp

like

--  The results of the following two statements are exactly the same 
select * from students where s_name = ' Zhang San ';
select * from students where s_name like ' Zhang San '; -- like Only collocation % and _ Only then and = Dissimilarity
-- The following sentence filters out students surnamed Zhang , such as ' Zhang San ',' zhang wuji ' etc. , It can also match ' Zhang '.
select * from students where s_name = ' Zhang %';
-- Can match ' Zhang San '、' Zhang Fei ' etc. , But it doesn't match ' zhang wuji '.
select * from students where s_name = ' Zhang _';

% and _ It's all wildcards , The former matches any character of any length , Include 0 length , The latter matches a single arbitrary character .

regexp

except like This simple screening condition , We can still do that MySQL Regular expressions are used as filters in

select prod_name from products where regexp 'ipad air[12]';
-- You can find the trade name :'ipad air1 64g'、'ipad air2 128g'、'ipad air1 128g' etc.

The knowledge of regular expressions is not detailed here , Please refer to the relevant materials for study .

What needs to be explained :

  • MySQL Regular expressions in Escape character by \\.
  • Be careful like and regexp The important difference between , The former is a perfect match for Strings , The latter is partial matching , Regular is a perfect match only if the first locator is added .
  • MySQL Of 3.23.4 Later versions are case insensitive in regular expressions ! If you need to be case sensitive, use regexp binary

Text processing function

Text processing functions can be used in filtering conditions

function explain
Left() Returns the character to the left of the string
Right() Returns the character to the right of the string
Locate() Find a substring of a string
SubString() Return the character of the substring
LTrim() Remove the space to the left of the string
RTrim() Returns the character to the right of the string
Length() Returns the length of the string
Soundex() Return string SOUNDEX value
Lower() Convert string to lowercase
Upper() Convert string to uppercase
--  Example : find 'tom','Tom','TOM' etc. 
select name from students where Lower(name) = 'tom';

Numerical processing function

function explain
Abs() Returns the absolute value of a number
Mod() Returns the remainder of the division operation
Exp() Returns the index value of a number
Mod() Returns the remainder of the division operation
Sin() Returns the sine of an angle
Cos() Returns the cosine of an angle
Tan() Returns the tangent of an angle
Sqrt() Returns the square root of a number
Pi() Return the PI
Rand() Returns a random number
--  Example : Find out if all the scores are in single digits 5 Student information , such as 75、85、65 etc. 
select * from students where Mod(score,10) = 5;

Date processing function

function explain
AddDate() Add a date ( God 、 Zhou et al )
AddTime() Add a time ( when 、 Grading )
CurDate() Return current date
CurTime() Return current time
Date() Return date part of date time
DateDiff() Calculate the difference between the two dates
Date_Add() Highly flexible date operation function
Date_Format() Returns a formatted date or time string
Day() Returns the days part of a date
DayOfWeek() For a date , Returns the corresponding day of the week
Hour() Returns the hour part of a time
Minute() Returns the minute part of a time
Month() Returns the month part of a date
Now() Returns the current date and time
Second() Returns the second part of a time
Time() Returns the time part of a date time
Year() Returns the year part of a date
--  Example : Find out all 1999 Information about students born in 
select * from students where Year(birthday) = 1999;

<!> Be careful stay MySQL The assignment format for the date type in is yyyy-mm-dd,datetime The type contains not only the date but also the time , So if you want to filter on 1999-07-24 When using birthday = '1999-07-42' It's not accurate , This only matches 00:00:00 Time . belong birthday Put it on the outside Date() function . When dealing with dates, we should make good use of date processing functions !

between

be used for Numerical range or Time interval

--  Example : Find out all 90 After the information of students 
select * from students where Date(birthday) between '1990-01-01' and '1990-12-31';

in

in I can represent the range , A collection, to be exact

  • Use a : parentheses
--  Example : Find out all ages for 15、18 or 24 Information about our students 
select * from students where age in (15,18,24);
  • Use two : Subquery
--  Suppose there is a table items Store product information , A table orders Store order information ( Suppose an order has only one product )
-- Example : Find out all the purchases ipad air Orders for a range of products
select * from orders where orders.item_id in (
select item_id from items where item_name regexp 'ipad air'
);

Null check

null Values in MySQL The Chinese price is special , It represents an unspecified or uncertain value . Whether they have null You need to use is null To judge

--  Example : Find out all the unfilled profile information ( The value must be null, Empty strings don't count , Just spaces are not ) My book book_id
select book_id from books where info is null;

<!> Be careful NULL Does not match

When you filter out rows that do not have a specific value , You may want to return with NULL Row of values . however , no way . Because the unknown has a special meaning , The database doesn't know if they match , So don't return them when they match or don't match filtering . therefore , When filtering data , Be sure to verify that the returned data does give that the filtered column has NULL The line of .

summary

  • Comparative relationship : Greater than 、 Less than 、 be equal to ...
  • and、or、not
  • String filtering :like、 Regular 、 Text processing function
  • function : Numerical processing 、 Date processing 、 Text processing
  • between、in、 Control checks

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