Cattle off network SQL combat two brush

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Cattle off network SQL combat two brush | Day1

Recently started to review and consolidate SQL, doing every day doing notes in a word document, found that just 10 days after brushing all 61 exercises. Brush title two intend to re-organize topics and ideas, fought blog notes. This is for your own study notes, but also hope that're reading this blog you also find it useful ~

The format of each note roughly, three major sections:

Outline
Title (Title Description, ideas, codes, reference)
review

Topic Source: https://www.nowcoder.com/ta/sql then we start Day 1 notes it!

First, outline

Question number	Knowledge Point
1	MAX () function
2	LIMIT and OFFSET usage
3	INNER JOIN to join two tables
4	INNER JOIN to join two tables
5	LEFT JOIN to join two tables
6	INNER JOIN / parallel query / ORDER BY reverse order

Second, the title

1. Find all the information at the latest recruits

Title Description

Find all the information at the latest recruits of
the CREATE TABLE employees(
emp_noint (11) the NOT NULL,
birth_dateDATE the NOT NULL,
first_nameVARCHAR (14) the NOT NULL,
last_nameVARCHAR (16) the NOT NULL,
genderchar (1) the NOT NULL,
hire_dateDATE the NOT NULL,
PRIMARY KEY ( emp_no));

Output Description

emp_no	birth_date	first_name	last_name	gender	hire_date
10008	1958-02-19	Sniy	Kalloufi	M	1994-09-15

Ideas
with MAX () function to find the "latest" record.
Code

SELECT * FROM employees
WHERE hire_date = (SELECT MAX(hire_date) FROM employees)

MAX () function usage reference
https://www.runoob.com/sql/sql-func-max.html

2. Find recruits ranked third time information of all employees

Title Description

Find recruits time ranked third from the staff all the information
the CREATE TABLE employees(
emp_noint (11) the NOT NULL,
birth_dateDATE the NOT NULL,
first_nameVARCHAR (14) the NOT NULL,
last_nameVARCHAR (16) the NOT NULL,
genderchar (1) the NOT NULL,
hire_dateDATE the NOT NULL ,
a PRIMARY KEY ( emp_no));

Output Description

emp_no	birth_date	first_name	last_name	gender	hire_date
10005	1955-01-21	Kyoichi	Maliniak	M	1989-09-12

Thinking

With LIMIT and OFFSET find the "bottom third" records;
Note that, with distinct deduplication, because there may be multiple duplicate date.

Code

SELECT * FROM employees 
WHERE hire_date = 
(SELECT DISTINCT hire_date FROM employees ORDER BY hire_date DESC LIMIT 1 OFFSET 2)

Limit the use of the offset for use with
for example, LIMIT 3 OFFSET 1which means, skip recording article (i.e., starting from the second record), and returns the next three records. That is ultimately obtained, the original clause 2,3,4 record.

Reference: https://www.runoob.com/sqlite/sqlite-limit-clause.html

3. Find the current salary details, and department number dept_no

topic

Find (to_date = '9999-01-01') leading the current salary details of the various departments and their counterparts in the current number dept_no
the CREATE TABLE dept_manager(
dept_nochar (4) the NOT NULL,
emp_noint (11) the NOT NULL,
from_dateDATE the NOT NULL,
to_dateDATE the NOT NULL,
KEY a PRIMARY ( emp_no, dept_no));
the CREATE TABLE salaries(
emp_noint (. 11) the NOT NULL,
salaryint (. 11) the NOT NULL,
from_dateDATE the NOT NULL,
to_dateDATE the NOT NULL,
a PRIMARY KEY ( emp_no, from_date));

Output Description

emp_no	salary	from_date	to_date	dept_no
10002	72527	2001-08-02	9999-01-01	d001
10004	74057	2001-11-27	9999-01-01	d004
10005	94692	2001-09-09	9999-01-01	d003
10006	43311	2001-08-02	9999-01-01	d002
10010	94409	2001-11-23	9999-01-01	d006

Thinking

The two tables are connected (inner join), by association emp_no
Two "current" - the current leadership, current salary, with to_date = "9999-01-01" limit

Code

SELECT s.*, dm.dept_no
FROM salaries AS s INNER JOIN dept_manager AS dm
ON s.emp_no = dm.emp_no 
WHERE s.to_date = '9999-01-01' AND dm.to_date = '9999-01-01'

Several connections on

this map Figure Wayne expressed very clearly, before I excerpt of the notes, now can not find the source, and the source of the network.

4. Find all the staff have been assigned sectors last_name and first_name

topic

Find employees all have been assigned sectors last_name and first_name
the CREATE TABLE dept_emp(
emp_noint (11) the NOT NULL,
dept_nochar (4) the NOT NULL,
from_dateDATE the NOT NULL,
to_dateDATE the NOT NULL,
PRIMARY KEY ( emp_no, dept_no));
the CREATE TABLE employees(
emp_noint (11) NULL the NOT,
birth_dateDATE the NOT NULL,
first_nameVARCHAR (14) the NOT NULL,
last_nameVARCHAR (16) the NOT NULL,
genderchar (. 1) the NOT NULL,
hire_dateDATE the NOT NULL,
a PRIMARY KEY ( emp_no));

Output Description

last_name	first_name	dept_no
Facello	Georgi	d001
Omission	Omission	Omission
Piveteau	Duangkaew	d006

Ideas
To screen out "has been assigned sector employees", two tables can be connected with the INNER JOIN keyword emp_no. 3 similar problems.
Code

SELECT e.last_name, e.first_name, de.dept_no
FROM employees AS e
INNER JOIN dept_emp AS de ON e.emp_no = de.emp_no

5. Find all employees last_name and first_name and the corresponding department number dept_no

topic

Find all employees last_name and first_name and the corresponding department number dept_no, but also to show not assigned a specific sector employees
the CREATE TABLE dept_emp(
emp_noint (11) the NOT NULL,
dept_nochar (4) the NOT NULL,
from_dateDATE the NOT NULL,
to_dateDATE the NOT NULL,
PRIMARY KEY ( emp_no, dept_no));
the CREATE TABLE employees(
emp_noint (. 11) the NOT NULL,
birth_dateDATE the NOT NULL,
first_nameVARCHAR (14) the NOT NULL,
last_nameVARCHAR (16) the NOT NULL,
genderchar (. 1) the NOT NULL,
hire_dateDATE the NOT NULL,
a PRIMARY KEY ( emp_no));

Output Description

last_name	first_name	dept_no
Facello	Georgi	d001
Omission	Omission	Omission
Lock	Mary	NULL (in sqlite in here is empty, MySQL is NULL)

Ideas
different from the title 4, "but also did not show the distribution sector-specific staff," this time with a LEFT JOIN to join two tables. Table dept_emp left connection table employees, the employees table will return all the data, even if no corresponding data table dept_emp.

JOINS usage can be viewed topic references 3.
Code

SELECT e.last_name, e.first_name, de.dept_no
FROM employees AS e
LEFT JOIN dept_emp AS de ON e.emp_no = de.emp_no

6. Find the salaries of all staff induction time

topic

查找所有员工入职时候的薪水情况，给出emp_no以及salary，并按照emp_no进行逆序
CREATE TABLE employees (
emp_no int(11) NOT NULL,
birth_date date NOT NULL,
first_name varchar(14) NOT NULL,
last_name varchar(16) NOT NULL,
gender char(1) NOT NULL,
hire_date date NOT NULL,
PRIMARY KEY (emp_no));
CREATE TABLE salaries (
emp_no int(11) NOT NULL,
salary int(11) NOT NULL,
from_date date NOT NULL,
to_date date NOT NULL,
PRIMARY KEY (emp_no,from_date));

输出描述

emp_no	salary
10011	25828
省略	省略
10001	60117

思路

用emp_no关键字连接两张表；
「所有员工入职时候的薪水情况」，即通过 employees表的hire_date = salaries表的from_date 限制；
「按照emp_no进行逆序」，即用ORDER BY 对emp_no进行排序。

代码

方法一：INNER JOIN连接两张表

SELECT s.emp_no, s.salary
FROM employees AS e INNER JOIN salaries AS s
ON e.emp_no = s.emp_no AND e.hire_date = s.from_date
ORDER BY s.emp_no DESC

方法二：并列查询

SELECT s.emp_no, s.salary
FROM employees AS e, salaries AS s
WHERE e.emp_no = s.emp_no AND e.hire_date = s.from_date
ORDER BY s.emp_no DESC

JOIN the INNER VS parallel query
Q: These two methods are different?
A: connection within the intersection after taking about two tables to form a new table, two tables are still tied with FROM or two tables. If you also want to use the new operating table within the connector. From an efficiency point of view should be parallel query FROM faster, because there is no formation of a new table. This problem no difference from the point of view the effect of two methods.

Source: Cattle customer network, Author: wasrehpic; https://www.nowcoder.com/questionTerminal/23142e7a23e4480781a3b978b5e0f33a

Third, review

It corresponds to the outline, combing classified Day1 of six questions again,

Question number	Knowledge Point
1	Use function MAX ()
2	Use clause LIMIT, with OFFSET
3、4、5、6	Table connection (INNER JOIN / LEFT JOIN)
6	ORDER BY clause to sort

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