nicolehhy / SQL-TIPS

There are some tips we should keep in mind when we use SQL language

SQL-TIPS

There are some tips we should keep in mind when we use SQL language

1. Tips in SELECT ( When we hvae subquery, we should use where to join two different tables)

• We can select COUNT() -/+ COUNT() in SQL
• We can select an output of subquery as our new columns

SELECT countries.name AS country, 
(SELECT COUNT(*) 
FROM cities
  WHERE countries.code = cities.country_code)   # make sure we join two tables using 'where'
AS cities_num.  # new column from another table
FROM countries
ORDER BY cities_num DESC, country
LIMIT 9;

• Subquery can be a new table

SELECT p.code,local_name,p.lang_num
From countries,    # table 1
	-- Subquery (alias as subquery)
	(SELECT code,count(*) AS lang_num
	 From languages
	 GROUP BY code) AS p.  # table 2
-- Where codes match
WHERE countries.code = p.code  # use where to limit the data
-- Order by descending number of languages
Order by p.lang_num DESC;

• Subquery can be a condition when we use WHERE

-- Select fields
SELECT name, continent, inflation_rate
-- From countries
FROM countries
  -- Join to economies
INNER JOIN economies
  -- Match on code
ON countries.code = economies.code
-- Where year is 2015
WHERE year = 2015
-- And inflation rate in subquery (alias as subquery)
AND inflation_rate IN (
      SELECT MAX(inflation_rate) AS max_inf
      FROM (
           SELECT name, continent, inflation_rate
           FROM countries
           INNER JOIN economies
           ON countries.code = economies.code
           WHERE year = 2015) AS subquery
      GROUP BY continent);           # here we use WHERE ... IN (subquery) to filter our data

2. Tips in WHERE ( When we need to filter the results, we will probably use 'where' clause)

• Where clause cannot be used with Aggregate functions(AVG,MIN,MAX,MEAN,SUM,COUNT)
• Sometimes we use where along with these signals

= #equal
<> #not equal
< #less than
> #greater than
<= #less than or equal to
>= #greater than or equal to 

• When we have AND and OR in the where clause, be sure to enclose the individual clauses in parentheses, like so:

SELECT title
FROM films
WHERE (release_year = 1994 OR release_year = 1995)  # here we can also use ' where year between 1994 and 1995' 
AND (certification = 'PG' OR certification = 'R');

3. Tips in missing value

• Sometimes, we need to query our data without NA, using IS NULL / IS NOT NULL

SELECT COUNT(*)
FROM people
WHERE birthdate IS/ IS NOT NULL;

• COUNT() doesn’t count the missing value of the entity you choose

4. Tips in UNION and UNION ALL

• Union: join two tables but it will delete the replicates with order
• Union All: just bind two tables together, including everything

5. Tips in UNION, INTERSECT, EXCEPT, MINUS and USING

we have to make sure the name of the fields we choose are the same among the tables we choose from, then we can use these command in our script

6. Tips in Scalar function

• Sometimes, we use mod(x,y) to filter our data(eg: even or odd number)

SELECT name 
FROM city
WHERE mod(code,2) = 1;

• The other scalar functions could be UCASE(), NOW(),ROUND()

SQL statements

The most commonly used SQL statements.

Basically used SQL statements

• QUERYING DATA FROM A TABLE

  • This part involves Select Where Group by Having Order by Limit usages.
  • This usages help us filter the data
  • Their order should be like :

    SELECT

    WHERE

    GROUP BY

    HAVING

    ORDER BY

• QUERYING FROM MULTIPLE TABLES

  • This part involves Join/Inner Join Left Join Right Join Full Outer Join Cross Join usages
  • This usages help us link multiple tables

• USING SQL OPERATORS

  • This part involves Union Except Intersect Minus usages.
  • This usages help us filter or bind the information we want efficiently
  • Keep in mind that when use this command, the entities of these tables should be the same

• MANAGING TABLES

  • This part involves Create Table Alter *table Drop Alter *table Add Alter *table Rename usages
  • This usages help us manage the tables in the database. We can delete/add the entities or rename them

• MODIFYING DATA

  • Insert multiple rows into a table

 INSERT INTO t(column_list) 
 VALUES (value_list),
(value_list), ....;

• Insert rows from t2 into t1

 INSERT INTO t1(column_list) 
 SELECT column_list
FROM t2;

• Delete subset of rows in a table

  DELETE FROM t
WHERE condition;

The best interview questions

Frequently asked SQL Interview Questions with detailed answers and examples.Here are many sample SQL Interview questions in a few PDF files and their answers are given just below to them. If you need them, just download the PDF files in this SQL repository.