[MySQL] official website document learning query statement SQL precautions

Preface

MySQL5.7 SELECT file .

• Test data scripts

CREATE TABLE `class` (
  `id` int(11) NOT NULL,
  `name` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

CREATE TABLE `student` (
  `id` int(11) NOT NULL,
  `name` varchar(255) CHARACTER SET latin1 DEFAULT NULL,
  `age` int(11) DEFAULT NULL,
  `score` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

INSERT INTO `james`.`class` (`id`, `name`) VALUES (1, ' Class one ');
INSERT INTO `james`.`class` (`id`, `name`) VALUES (2, ' Class two ');
INSERT INTO `james`.`class` (`id`, `name`) VALUES (3, ' Class three ');

INSERT INTO `james`.`student` (`id`, `name`, `age`, `score`) VALUES (1, 'james', 18, 60);
INSERT INTO `james`.`student` (`id`, `name`, `age`, `score`) VALUES (2, 'tom', 18, 80);
INSERT INTO `james`.`student` (`id`, `name`, `age`, `score`) VALUES (3, 'jerry', 20, 40);
INSERT INTO `james`.`student` (`id`, `name`, `age`, `score`) VALUES (4, 'candy', 16, 70);
INSERT INTO `james`.`student` (`id`, `name`, `age`, `score`) VALUES (5, 'kobe', 20, 60);

Query statement

Can be disabled group by The default sort of （ Reduce sorting overhead ）

original text ： If you use GROUP BY, output rows are sorted according to the GROUP BY columns as if you had an ORDER BY for the same columns. To avoid the overhead of sorting that GROUP BY produces, add ORDER BY NULL:

paraphrase ： Use group by when , The default will be for group by Column sorting （ As shown in Figure 2 , Press age Positive order ）. If you want to cancel this default behavior , You can add order by null.

select * from student group by age order by null;

stay selete Use in order by group by Don't ignore when sorting max_sort_length

original text ： When you use ORDER BY or GROUP BY to sort a column in a SELECT, the server sorts values using only the initial number of bytes indicated by the max_sort_length system variable.

paraphrase ： Use order by or group by When sorting , The length used for sorting in the field is less than the global variable max_sort_length Specified number of bytes （ Unit is bytes）.

show variables like 'max_sort_length';

group by [ ] with rollup Bring out the statistics

HAVING Precautions for

original text ：The HAVING clause, like the WHERE clause, specifies selection conditions. The WHERE clause specifies conditions on columns in the select list, but cannot refer to aggregate functions. The HAVING clause specifies conditions on groups, typically formed by the GROUP BY clause. The query result includes only groups satisfying the HAVING conditions. (If no GROUP BY is present, all rows implicitly form a single aggregate group.)

paraphrase （ It's about understanding ）：

• where Is to filter the returned result set , The implication is ：where stay select Execute... Before statement execution , If there is group by sentence ,where You can only see the result set after grouping , Unable to read the details of each group before grouping .
• having It is used to filter the grouped result set , The implication is ：having You can use aggregate functions to write conditions , You can also write conditions without using aggregate functions （ amount to where The function of , But it will cause ambiguity ）.

--  Students are grouped by age , And as long as the average score is greater than 50 Group 
select age from student group by age having avg(score) > 50;

• because as Operation in where After performing , The same requirement cannot be implemented with the following statement

select age, avg(score) as b from student group by age where b > 50; //  error 

• where It is independent of the aggregate function , So you can't use an aggregate function as a statement

select age from student group by age where avg(score) > 50; //  error 

• having Not with group by Together with , It can be executed , If having Do not use aggregate function after , Semantics and where Agreement , It is suggested to change it to where.

select * from student having score > 50; //  wrong 
 --  Modified into where
select * from student where score > 50; 

• having Not with group by Together with , If at this time having Using aggregate functions , Treat all result sets as a group ;
  

limit Note the semantics of the two parameters

• Before acquisition n rows

select * from student limit 3;

• The offset from is 3 Start of record , Take back 1 Bar record
  The offset of the first line is 0, So it's from id=4 Start with
  

Specify index explicitly index hints

MySQL5.7 index hints Used to declare the index used by the query （ Not necessarily adopted by the query optimizer ）

SELECT * FROM table1 USE INDEX (col1_index,col2_index) WHERE col1=1 AND col2=2 AND col3=3;

SELECT * FROM table1 IGNORE INDEX (col3_index) WHERE col1=1 AND col2=2 AND col3=3;

SELECT * FROM t1 USE INDEX (i1) IGNORE INDEX FOR ORDER BY (i2) ORDER BY a;

original text ： Index hints apply to SELECT and UPDATE statements. They also work with multi-table DELETE statements, but not with single-table DELETE,
paraphrase ：index hints The grammar of can be used in select and update On , If and only if multiple tables participate （ Subquery or connection ） Can only be used in delete On

Do not use column subscripts

• group by / order by You can use aliases in statements , You can even use subscripts , It is worth mentioning that the subscript of the column is from 1 Start , But don't do it

SELECT college, region, seed FROM tournament ORDER BY 2, 3;

original text ： Use of column positions is deprecated because the syntax has been removed from the SQL standard.

paraphrase ： The use of subscripts has been sql Standard deletion