(Continuous update) 2020-12-31-SQL (SQLZOO SELECT FROM WORLD) Rewriting notes-HK

SQLZOO-SELECT FROM WORLD

Small catalog

• Subquery returns more than 1 row error resolution;
• How to express a percentage with a percent sign;
• Do "comparison operation" with NULL value, resulting in a null output value;
• Two methods to find the maximum value (max, min);

text

1. There are multiple sub-queries that meet the conditions, and a sequence is returned, and a continent variable cannot be compared with a bunch of return values ​​at the same time, resulting in an error

error: Subquery returns more than 1 row
Solution: add any keyword before the subquery

select name, continent
from world
where continent = any(
  select distinct continent from world
  where name = 'Argentina' or name = 'Australia'
)
order by name

2. Representation of an integer percentage with a percent sign
   Use round() to make a rounding truncation, and then use concat() to splice% signs;
   the precision of the percentage can be adjusted, just adjust the B parameter of round(A, B).

select name, 
       concat(round(100*population/(select population 
                                      from world 
                                      where name = 'Germany'))
       , '%') as percentage
from world
where continent = 'Europe'

3. When the number to the right of the comparison sign is NULL, there will be no output value.

Example:
This code does not exclude countries with gdp=NULL. The all() function will cause gdp to be compared with NULL, causing the output value to be lost;

select name 
from world
where gdp > all(select gdp 
                  from world 
                 where continent = 'Europe' )

In addition to the restriction that gdp is a non-negative number, it is actually an operation to remove the NULL value, and the result can be output normally.

select name 
from world
where gdp > all(select gdp 
                  from world 
                 where continent = 'Europe' 
                 && gdp>=0)

4. When looking for the best value,
   you can use >= to compare with yourself;

SELECT continent, name, area 
FROM world x
WHERE area >= ALL(SELECT area 
                   FROM world y
                  WHERE y.continent=x.continent
                 && area>0)

You can also limit it in where not to compare with yourself;

SELECT continent, name, area 
FROM world x
WHERE area > ALL(SELECT area 
                   FROM world y
                  WHERE y.continent=x.continent
                 && area>0
                 && x.name <> y.name)