05, DQL: query records in the table

* select * from 表名;

1. Syntax:
   select
       field list
   from
       table name list
   where
       condition list
   group by
       grouping field
   having
       grouping conditions after grouping
   order by
       sorting
   limit
       paging limit

2. Basic query
   1. Multi-field query
       select field name 1, field name 2... from table name;
       * Note:
           * If you query all fields, you can use * to replace the field list.
   2. Removal of duplication:
       * distinct
   3. Calculating columns
       * Generally, four arithmetic operations can be used to calculate the values of some columns. (Generally only numerical calculations are performed)
       * ifnull (expression 1, expression 2): the calculation that null participates in, the calculation result is all null
           * expression 1: which field needs to be judged whether it is null
           * if the field is null The replacement value after.
   4. Alias:
       * as: as can also be omitted
3. Conditional query
   1. Where clause followed by condition
   2. Operator
       *>, <, <=, >=, =, <>
       * BETWEEN...AND
       * IN (Collection)
       * LIKE: fuzzy query
           * Placeholder:
               * _: a single arbitrary character
               * %: Multiple arbitrary characters
       * IS NULL
       * and or &&
       * or or ||
       * not or!

 -- 查询年龄大于20岁

            SELECT * FROM student WHERE age > 20;
            
            SELECT * FROM student WHERE age >= 20;
            
            -- 查询年龄等于20岁
            SELECT * FROM student WHERE age = 20;
            
            -- 查询年龄不等于20岁
            SELECT * FROM student WHERE age != 20;
            SELECT * FROM student WHERE age <> 20;
            
            -- 查询年龄大于等于20 小于等于30
            
            SELECT * FROM student WHERE age >= 20 &&  age <=30;
            SELECT * FROM student WHERE age >= 20 AND  age <=30;
            SELECT * FROM student WHERE age BETWEEN 20 AND 30;
            
            -- 查询年龄22岁，18岁，25岁的信息
            SELECT * FROM student WHERE age = 22 OR age = 18 OR age = 25
            SELECT * FROM student WHERE age IN (22,18,25);
            
            -- 查询英语成绩为null
            SELECT * FROM student WHERE english = NULL; -- 不对的。null值不能使用 = （!=） 判断
            
            SELECT * FROM student WHERE english IS NULL;
            
            -- 查询英语成绩不为null
            SELECT * FROM student WHERE english  IS NOT NULL;

-- 查询姓马的有哪些？ like
			SELECT * FROM student WHERE NAME LIKE '马%';
			-- 查询姓名第二个字是化的人
			
			SELECT * FROM student WHERE NAME LIKE "_化%";
			
			-- 查询姓名是3个字的人
			SELECT * FROM student WHERE NAME LIKE '___';

-- 查询姓名中包含德的人
			SELECT * FROM student WHERE NAME LIKE '%德%';

05, DQL: query records in the table

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