SQL笔记

• Limit Offset
  return only 10 records, start on record 16 (OFFSET 15)
  SELECT * FROM Orders LIMIT 10 OFFSET 15
• Order
  order by points desc, goal_diff desc, team_name;

• Join
  can have >, < conditions in join

  SELECT
       a.NAME AS Employee
  FROM Employee AS a JOIN Employee AS b
       ON a.ManagerId = b.Id
       AND a.Salary > b.Salary
  ;
  

• having

  select Email
  from Person
  group by Email
  having count(Email) > 1;
  

• window
  rank() over (partition by departmentId order by salary desc) as salaryRank
  dense_rank()
  lead(num,1) over(order by id) as next_num_1
  lead(num,2) over(order by id) as next_num_2
  lag(temperature) over (order by recordDate) as previous_temperature
• CTE common table expression
  with t1 as (...)
  select * from t1

with t1 as (...),
t2 as ()
select * from t1
union all
select * from t2

• Recursive CTE
  WITH RECURSIVE CTE AS (...)

• IFNULL / coalesce
  IFNULL(...,default_value_if_null)
  SELECT

    IFNULL(
      (SELECT DISTINCT Salary
       FROM Employee
       ORDER BY Salary DESC
        LIMIT 1 OFFSET 1),
    NULL) AS SecondHighestSalary
  

coalesce(round(avg(accepted),2),0)

• case
  select

    case when MOD(id,2)=1 and id=(select max(id) from Seat) then id 
     when MOD(id,2)=1 then id+1 
     when MOD(id,2)=0 then id-1 
     end as new_id 

  from Seat

• Delete

  DELETE p1 FROM Person p1,
      Person p2
  WHERE
      p1.Email = p2.Email AND p1.Id > p2.Id
  

• Update

  update Salary
  set sex = case when sex='m' then 'f' else 'm' end
  

• Other
  Request_day BETWEEN '2013-10-01' AND '2013-10-03'
  cast(count(*) as float)
  ROUND(COUNT(IF(Status != 'completed', TRUE, NULL)) / COUNT(*), 2)
  Datediff(recordDate,previous_recordDate)=1
  Date(call_time)
  year(order_date)=2019
  MOD(id,2)=1
  substring(phone_number,1,3)
  Subtract 10 days from a date and return the date: DATE_SUB("2017-06-15", INTERVAL 10 DAY)
  LEAST(x,y): return the minimum of x and y
  GREATEST(x,y): return the maximum of x and y
  SELECT GROUP_CONCAT(DISTINCT product ORDER BY product ASC SEPARATOR ',') AS products

• Function

  CREATE FUNCTION getNthHighestSalary(N INT) RETURNS INT
  BEGIN
  Declare n int default N-1;
    RETURN (
        # Write your MySQL query statement below.
        select distinct salary 
        from employee
        order by salary desc
        limit 1 offset n
    );
  END
  
  

• Pivot
  CREATE PROCEDURE PivotProducts()
  BEGIN

  #Override GROUP_CONCAT length which has a default limit of 1024
  SET SESSION group_concat_max_len = 1000000;

  #Store case statement for dynamically generated columns in a variable ie case_stmt
  SET @case_stmt = NULL;
  SELECT GROUP_CONCAT(DISTINCT CONCAT('SUM(CASE WHEN store = "', store, '" THEN price END) AS ', store))
  INTO @case_stmt
  FROM products;

  #Insert above statement (@case_stmt) in the following main query to frame final query
  SET @sql_query = CONCAT('SELECT product_id, ', @case_stmt, ' FROM products GROUP BY product_id');

  #Execute final query
  PREPARE final_sql_query FROM @sql_query;
  EXECUTE final_sql_query;
  DEALLOCATE PREPARE final_sql_query;

  END

Leetcode

(262. Trips and Users
(2252. Dynamic Pivoting of a Table
(1384. Total Sales Amount by Year
(2153. The Number of Passengers in Each Bus II