Flow control statement of SQL data analysis [if, case... When detailed]

zero 、 Write it at the front

All the code in this article is in SQL ZOO platform Conduct , Data is also available under the platform world surface , All the codes have passed the test .

One 、if Single condition judgment statement

1、 Grammatical structure ：if( Judge the condition , Establishment value , Invalid value )

select if(1=1,'Yes','No');

2、 It is judged that the soil area of China in Asia and Africa is larger than 200 Is a country with a large population （ The population is larger than 2 Billion ）

select continent,name,if(population>200000000,' A large population ',' Small country ')  Population size  
from world
where continent in ('Asia','Africa') and area>2000000
order by continent

Execution results ：

3、 Query land area is larger than 100 The overall economic level of million Asian countries （ Greater than 1 Trillion is a high economy ）

select name,gdp,if(gdp>1000000000000,' High economy ',' Low economies ') as  General economy 
from world
where continent='Asia' and area>1000000

Two 、case…when Multi condition judgment

1、 Single condition （if…else effect ）

Put the above if Modified into case…when：

# It is judged that the soil area of China in Asia and Africa is larger than 200 Is a country with a large population （ The population is larger than 2 Billion ）
select continent,name,
CASE
WHEN population>200000000 THEN ' A large population '
ELSE ' Small country ' 
END AS  Population size  
from world
where continent in ('Asia','Africa') and area>2000000
order by continent

# Query land area is larger than 100 The overall economic level of million Asian countries （ Greater than 1 Trillion is a high economy ）
select name,gdp,
CASE
when gdp>1000000000000 then ' High economy '
else ' Low economies '
END as  General economy 
from world
where continent='Asia' and area>1000000

2、 Multiple conditions （if…else if…else effect ）

from gdp It is roughly judged that the land area is larger than 50 ten thousand , The population is larger than 2000 The state of ten thousand Asian and European countries ：

select continent,name,population,gdp,
CASE
WHEN gdp>=3000000000000 THEN ' First class big country '
WHEN gdp>=2000000000000 and gdp<=3000000000000 THEN ' A second-class power '
WHEN gdp>=200000000000 and gdp<=1000000000000 THEN ' Third class big country ' 
ELSE ' Large developing countries '
END AS  State of the country  
from world
where continent in ('Asia','Europe') and area>500000 and population>20000000
order by continent

3、 Constant usage （switch…case effect ）

case The field or expression to determine
when Constant 1 then The value to display 1 Or words 1
when Constant 2 then The value to display 2 Or words 2
…
else The value to display n Or words n;

for example ： Make the population larger than 5000 The continent names of ten thousand countries are translated into the corresponding Chinese

select name,population,
CASE continent
WHEN 'Asia' THEN ' Asia '
WHEN 'Afica' THEN ' Africa '
WHEN 'South America' THEN ' South America '
WHEN 'North America' THEN ' In North America '
WHEN 'Europe' THEN ' The European '
ELSE ' Other continents '
END AS  Continent 
from world
where population>=50000000

3、 ... and 、ifnull Fill in empty values

Ifunull( Field name ,“ value 1”) If the content of this column is null, Then use ’' Fill in . If not null Words , Then the original value is displayed .

select id, ifnull('score',' miss an exam ') as  fraction ;

If the score is null, It means that the student is absent .