MySQL data query (where)

        WHERE The clause must be immediately followed by FROM After Clause , stay WHERE clause , Use a condition from FROM Select a row from the intermediate result of the clause . Grammar format ：WHERE < Decision operation >

Decision operation ： The result is true、false or unknown

        WHERE Clauses will match FROM The intermediate result of the clause is judged row by row , When conditions are ture when , One line is included in WHERE Intermediate result set of clause .

        stay SQL in , Returns the logical value （true or false） Operators or keywords of can be called predicates , Decision operation includes comparison operation 、 Pattern matching 、 Range comparison 、 Null value comparison and subquery .

 1、 Comparison operations

         The comparison operator is used to compare two expression values ,MySQL The supported comparison operators are ：=（ be equal to ）,<（ Less than ）,<=（ Less than or equal to ）,>（ Greater than ）,>=（ Greater than or equal to ）,<=>（ Equal or equal to nothing ）,<>（ It's not equal to ）,!=（ It's not equal to ）.

        Expression is division TEXT and BLOB Expressions outside of type .

        When both expression values are not null （NULL） when , except “<=>” Operator , Other comparison operations return logical values true（ really ） or false（ false ）. When one of the two expression values is null or both are null , Will return unknown.

  example ： Inquire about Bookstore database book The title of the book in the table is “ Web programming ” The record of

SELECT *
FROM book
WHERE  Title =' Web programming ';

  example ： Inquire about Book The unit price in the table is greater than 30 Books of

SELECT *
FROM Book
WHERE  The unit price >30;

         MySQL There is one special The equal operator of “<=>”, When two expressions are equal to each other or both are equal to null , The value of the operation is true, When there is a null value or both non null values but not equal, the result of this condition is false, But not unknown The situation of .

 2、 Logical operations

        Logical operation can pass the results of multiple decision operations through logical operators （AND、OR、XOR and NOT） Form more complex query conditions .

        Logical operators are used to test a condition , The result of operation is true（1） or false（0）.MySQL The logical operators provided are ：

not or ！： Logic is not

or or ||： Logic or

and or &&： Logic and

xor： Logical XOR

        The result of logical operation is 1 or 0, respectively true and false.

3、 Pattern matching  

        LIKE The operator is used to indicate whether a string matches the specified string , Its operands can be char、varchar、text、datetime And so on , Returns the logical value true or false.

Grammar format ： expression [not] LIKE expression

        Use LIKE When pattern matching , Special symbols are often used _ and % Fuzzy query ,% representative 0 Characters or more ,_ Single character .

        because MySQL Case insensitive by default , To be case sensitive, you need to change the proofing rules of the character set .

  example ： Inquire about Members Last name in the table “ Zhang ” Member's ID number 、 full name 、 Registration time

SELECT  ID number , Membership name , Registration time 
FROM Members
WHERE  Membership name  LIKE' Zhang %';

  example ： Inquire about book The book number in the table is the penultimate 2 Position as D Book number and title

SELECT  Book number , Title 
FROM book
WHERE  Book number  LIKE'%D_';

  To find one or all of the special symbols （_ and %）, An escape character must be used . When looking for underscores _ when , have access to ESCAPE'#' To define # For escape characters , such , In the statement # hinder _ It loses its original special significance , Underline that is considered normal _.

example ： Inquire about book Books with underlined titles in the table .

SELECT  Book number , Title 
FROM book
WHERE  Title  LIKE'%#_%'ESCAPE'#';

 4、 Range comparison

        There are two keywords used for range comparison , Namely BETWEEM and IN

        When the condition to be queried is a range of values , have access to BETWEEN keyword .BETWEEN Keywords indicate the query scope , The format is ： expression [NOT] BETWEEN expression 1 AND expression 2

example ： Inquire about Book In the table 2003 The situation of books published in  

SELECT *
FROM Book
WHERE  Publication date  BETWEEN'2003-1-1' AND '2003-12-31';

# or 
SELECT *
FROM Book
WHERE  Publication date >='2003-1-1' AND  Publication date <='2003-12-31';

  example ： To find out Book It's not in the table 2003 The situation of all books published in , Use NOT

SELECT *
FROM Book
WHERE  Publication date  NOT BETWEEN '2003-1-1' AND '2003-12-31';

# or 
SELECT *
FROM Book
WHERE  Publication date <='2003-1-1' OR  Publication date >='2003-12-31';

        Use IN Keywords can specify a value table , All possible values are listed in the value table , When it matches any one in the value table , Return true, Otherwise return to false.

Grammar format ： expression [NOT] IN( Subquery | expression 1[,... expression n])

        IN The most commonly used keywords are expression sub queries , It can also be used for OR operation .

  example ： Inquire about Book In the table “ Higher Education Press ”,“ Peking University press ” and “ People's University Press ” Published books

SELECT * FROM Book
WHERE  Press.  IN（' Higher Education Press ',' Peking University press ',' People's University Press '）;

# or 
SELECT * FROM Book
WHERE  Press. =' Higher Education Press '
OR  Press. =' Peking University press '
OR  Press. =' People's University Press ';

5、 Empty value comparison

        When it is necessary to determine whether the value of an expression is null , Use IS NULL keyword .

Grammar format ： expression IS [NOT] NULL

If the value of the expression is null , return true, Otherwise return to false, When using not when , The result is just the opposite .

example ： Inquire about Sell Order records in the table that have not been shipped

SELECT *
FROM Sell
WHERE  Whether to ship the goods or not  IS NULL;