sqlite & postgreSQL

Posted 2021-04-30 sshcmd学python

下载 postgreSQL

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cmd  已经安装anaconda
pip3 install psycopg2
以便 jupyter 或者 python3 中 import psycopg2

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先复习这点

GROUP BY   &  DISTINCT

The GROUP BY SQL statement allows us to compute summary statistics by “group,” or unique value. When we use this statement, SQL creates a group for each unique value in a column or set of columns (the same values we get when we use the DISTINCT statement), and then does the calculations for them.

When we want to filter on a column generated by a GROUP BY query, we can use the HAVING statement.

SELECT Major_category,AVG(ShareWomen) FROM recent_grads GROUP BY Major_category

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SELECT Major_category, AVG(Employed) / AVG(Total) AS share_employed FROM recent_grads GROUP BY Major_category;

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HAVING

SELECT Major_category ,AVG(Low_wage_jobs)/AVG(Total) share_low_wage FROM recent_gradsGROUP BY Major_category HAVING share_low_wage >.1

ROUND

SELECT ROUND(ShareWomen, 4),Major_category FROM recent_gradsLIMIT 10

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SELECT Major_category ,ROUND((AVG(College_jobs)/AVG(Total)),3)  share_degree_jobs FROM recent_grads GROUP BY Major_category HAVING share_degree_jobs<.3

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CAST

SELECT CAST(Women as Float) / CAST(Total as Float) FROM recent_grads limit 5

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Subqueries   run first

SELECT Major,Unemployment_rate FROM recent_grads WHERE Unemployment_rate< (SELECT AVG(Unemployment_rate) FROM recent_grads)ORDER BY Unemployment_rate

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Subquery in SELECT

SELECT COUNT(*), (SELECT COUNT(*) FROM recent_grads) FROM recent_gradsWHERE ShareWomen > (SELECT AVG(ShareWomen) FROM recent_grads)

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SELECT  (CAST(COUNT(*) as float)  /  CAST((SELECT COUNT(*) FROM recent_grads)as float))AS proportion_abv_avgFROM recent_gradsWHERE ShareWomen > (SELECT AVG(ShareWomen) FROM recent_grads);

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IN

SELECT Major, Major_category FROM recent_gradsWHERE Major_category IN ('Business', 'Engineering')LIMIT 7;

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SELECT Major, Major_category FROM recent_gradsWHERE Major_category IN (select Major_category from recent_gradsgroup by Major_categoryorder by SUM(Total) DESClimit 5)；

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Integrating a subquery with the outer query

select AVG(           cast(Sample_size as float)/cast(Total as float) ) avg_ratio from recent_grads;

select Major, Major_category, cast(Sample_size as float)/cast(Total as float) ratio from recent_gradswhere ratio > (select AVG(cast(Sample_size as float)/cast(Total as float)) avg_ratiofrom recent_grads);

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