MySQL learning notes - single table query

1. Select statement The SELECT Statement：

SELECT yes Column / Field Select statement , Selectable column , Mathematical expression between columns , Specific value or text , You can use AS Keyword to set column aliases （AS Omission ）, Be careful DISTINCT Use of keywords .

Be careful

SQL Will completely ignore case （ Case in most cases ）、 Extra space （ More than one space ）、 Indent and wrap ,SQL Statements are completely separated by semicolons ; Division , Indent with 、 Line breaks are just to make the code look more beautiful and the structure clearer , These and Python Very different , it is to be noted that .

2. WHERE

WHERE Is the row filter condition , Actually, it's line by line / One record after another verifies whether it meets the conditions , Screening

Comparison operator ：

> < = >= <=

!=/<> It's not equal to

3. AND,OR,NOT Operator

example

USE sql_store;

SELECT *
FROM customers
WHERE birth_date > '1990-01-01' AND points > 1000
/WHERE birth_date > '1990-01-01' OR 
      points > 1000 AND state = 'VA'

AND Priority over OR, But it's best to put parentheses , Clearer

WHERE birth_date > '1990-01-01' OR 
      (points > 1000 AND state = 'VA')

NOT Usage of

WHERE NOT (birth_date > '1990-01-01' OR points > 1000)

The parenthesized equivalent is converted to

WHERE birth_date <= '1990-01-01' AND points <= 1000

Attention priority ： mathematics → Compare → Logic

SELECT Clause ,WHERE Clause and the following ORDER BY Clauses, etc. can use mathematical expressions between columns

4. IN Operator

use IN Operator will be a certain attribute With multiple values （ A series of values ） Compare ; The essence is the simplification of multiple equal comparison operation conditions

use IN Operator simplifies the condition

WHERE state IN ('va', 'fl', 'ga')

coca NOT

WHERE state NOT IN ('va', 'fl', 'ga')

It's available here NOT Why ： You can see that ,IN sentence  IN ('va', 'fl', 'ga')  It's a judgment of whether it meets the conditions , It can be regarded as a special comparison operation , What you get is a logical value , So it can be used NOT To reverse

5. BETWEEN Operator

Used to express Range Type condition

Be careful

• use AND Not parentheses
• Closed interval , Include both ends
• It can also be used for dates , After all, dates are also numerical in nature , Dates also have sizes （ Sooner or later ）, Comparable operations
• Same as IN equally ,BETWEEN Essence is also a specific Simplification of multiple comparison operation conditions

6. LIKE Operator  

Fuzzy search , Find records of strings with certain patterns / That's ok

Be careful

• Outdated usage （ But sometimes it's easier to use , Later, I found that it seems to be used more ）, Regular expressions in the next lesson are more flexible and powerful
• Note that like regular expressions, strings are wrapped in quotation marks

USE sql_store;
SELECT * FROM customers
WHERE last_name like 'brush%' / 'b____y'

Describe the desired string pattern in quotation marks , Be careful SQL（ almost ） Case insensitive in any case

Two kinds of wildcards ：

• %  Any number （ Include 0 individual ） The characters of （ Use more ）any number of characters
• _  Single character single character

practice

Choose customers who meet the following conditions ：

1. The address contains 'TRAIL' or 'AVENUE'

2. The telephone number is in 9 end

USE sql_store;

select * 
from customers
where address like '%Trail%' or 
      address like '%avenue%'

LIKE The execution priority is after the logical operator , After all  IN BETWEEN LIKE The essence can be seen as the simplification of comparison operators , It should be at the same level as the comparison operation , mathematics → Compare → Logic , Always remember this order , The above one will be much simpler if regular expressions are used

where phone like '%9'
/where phone not like '%9'

LIKE The judgment result of is also a TRUE/FASLE The problem of , Any logical value / Brin value can be pre NOT To take the opposite

7. REGEXP Operator

Regular expressions , More powerful in searching strings , You can search for more complex templates

example

USE sql_store;

select * from customers
where last_name like '%field%'
--  Equivalent to 
where last_name regexp 'field'

regexp yes regular expression（ Regular expressions ） Abbreviation

Regular expressions can be combined to express more complex string patterns

where last_name regexp '^mac|field$|rose' 
where last_name regexp '[gi]e|e[fmq]' --  Find the ge/ie or ef/em/eq Of 
where last_name regexp '[a-h]e|e[c-j]'

Regular expression summary ：

Symbol	significance
^	start beginning
$	ending end
[abc]	contain abc
[a-c]	contain a To c
|	logical or

practice

Choose customers who meet the following conditions ：

1. first names yes ELKA or AMBUR

2. last names With EY or ON end

3. last names With MY start Or contain SE

4. last names contain BR or BU

select * 
from customers
where first_name regexp 'elka|ambur'
/where last_name regexp 'ey$|on$'
/where last_name regexp '^my|se'
/where last_name regexp 'b[ru]'/'br|bu'

8. IS NULL and IS NOT NULL Operator  

Find the null value , Find records with some missing attributes

Be careful

yes IS NULL and IS NOT NULL here NOT Not preceded by brin value , It is more in line with English grammar be After verb

9. ORDER BY Sort

Sort statement , and  SELECT ……  It's like ：

1. Multi column
2. It can be a mathematical expression between columns
3. Can include any column , Include unselected columns （MySQL characteristic , Other DBMS There may be a mistake ）,
4. It can be a previously defined alias column （MySQL characteristic , It can even be a column alias set with a constant ）
5. Any sort by column Can be added DESC

Be careful

Better not use  ORDER BY 1, 2（ Said to  SELECT ……  Select the... In the column 1、2 List as sorting basis ） This implicit basis , because SELECT When the selected column changes, it is easy to make mistakes , It is better to write the column names explicitly as the basis for sorting

notes ：workbench The middle wrench icon opens the design mode of the table , View or modify the columns in the table （ attribute ）, You can see who is the primary key . If the sorting statement is omitted, it will be sorted by primary key by default

10.  LIMIT Clause

Limit the number of records that return results ,“ front N individual ” or “ skip M The last one is the first one N individual ”

USE sql_store;

select * from customers
limit 3 / 300 / 6, 3

LIMIT 6, 3 Before you jump 6 individual , Take the first place 7~9 individual ,6 It's the offset ,
Such as ： Page paging Every time 3 Records show one page The first 3 The record that the page should display is limit 6, 3

Be careful

SELECT The sentence is over , The order of the clauses inside is fixed, and you should remember , If the order is disordered, an error will be reported
select from where + order by limit
Vertical selection column , Determination table , Select the row horizontally （ The writing and combination of various conditions should be clear and familiar ）, Finally, sort and limit