adiag321 / SQL-Practice-Questions

This repository contains solutions for various SQL websites

SQL-Practice-Questions

This repository contains solutions for various SQL websites -

• Hackerrank
• Interview Query
• SQLZoo -
• DataLemur
• LeetCode
• Stratascratch
• DataFord.io
• Leetcode SQL50
• Window Function

SQL NOTES:

• Finding Data Queries
• Data Modification Queries
• Reporting Queries
• Join Queries
• View Queries
• Altering Table Queries
• Creating Table Query

IMPORTANT SQL FUNCTIONS TO REMEMBER:

• String functions (CONCAT, LENGTH, SUBSTRING, REPLACE, UPPER, LOWER)
• Date and time functions (DATE, TIME, TIMESTAMP, DATEPART, DATEADD, DATA_FORMAT)
• Numeric functions (ROUND, CEILING, FLOOR, ABS, MOD)
• Conditional functions (CASE, COALESCE, NULLIF)

Finding Data Queries

SELECT: used to select data from a database

• SELECT * FROM table_name;

DISTINCT: filters away duplicate values and returns rows of specified column

• SELECT DISTINCT column_name;

WHERE: used to filter records/rows

• SELECT column1, column2 FROM table_name WHERE condition;
• SELECT * FROM table_name WHERE condition1 AND condition2;
• SELECT * FROM table_name WHERE condition1 OR condition2;
• SELECT * FROM table_name WHERE NOT condition;
• SELECT * FROM table_name WHERE condition1 AND (condition2 OR condition3);
• SELECT * FROM table_name WHERE EXISTS (SELECT column_name FROM table_name WHERE condition);

ORDER BY: used to sort the result-set in ascending or descending order

• SELECT * FROM table_name ORDER BY column;
• SELECT * FROM table_name ORDER BY column DESC;
• SELECT * FROM table_name ORDER BY column1 ASC, column2 DESC;

SELECT TOP: used to specify the number of records to return from top of table

• SELECT TOP number columns_names FROM table_name WHERE condition;
• SELECT TOP percent columns_names FROM table_name WHERE condition;
• Not all database systems support SELECT TOP. The MySQL equivalent is the LIMIT clause
• SELECT column_names FROM table_name LIMIT offset, count;

LIKE: operator used in a WHERE clause to search for a specific pattern in a column

• % (percent sign) is a wildcard character that represents zero, one, or multiple characters
• _ (underscore) is a wildcard character that represents a single character
• SELECT column_names FROM table_name WHERE column_name LIKE pattern;
• LIKE ‘a%’ (find any values that start with “a”)
• LIKE ‘%a’ (find any values that end with “a”)
• LIKE ‘%or%’ (find any values that have “or” in any position)
• LIKE ‘[a-c]%’ (find any values starting with “a”, “b”, or “c”

IN: operator that allows you to specify multiple values in a WHERE clause

• essentially the IN operator is shorthand for multiple OR conditions
• SELECT column_names FROM table_name WHERE column_name IN (value1, value2, …);
• SELECT column_names FROM table_name WHERE column_name IN (SELECT STATEMENT);

BETWEEN: operator selects values within a given range inclusive

• SELECT column_names FROM table_name WHERE column_name BETWEEN value1 AND value2;
• SELECT * FROM Products WHERE (column_name BETWEEN value1 AND value2) AND NOT column_name2 IN (value3, value4);
• SELECT * FROM Products WHERE column_name BETWEEN #01/07/1999# AND #03/12/1999#;

NULL: values in a field with no value

• SELECT * FROM table_name WHERE column_name IS NULL;
• SELECT * FROM table_name WHERE column_name IS NOT NULL;

AS: aliases are used to assign a temporary name to a table or column

• SELECT column_name AS alias_name FROM table_name;
• SELECT column_name FROM table_name AS alias_name;
• SELECT column_name AS alias_name1, column_name2 AS alias_name2;
• SELECT column_name1, column_name2 + ‘, ‘ + column_name3 AS alias_name;

UNION: set operator used to combine the result-set of two or more SELECT statements

• Each SELECT statement within UNION must have the same number of columns
• The columns must have similar data types
• The columns in each SELECT statement must also be in the same order
• SELECT columns_names FROM table1 UNION SELECT column_name FROM table2;
• UNION operator only selects distinct values, UNION ALL will allow duplicates

INTERSECT: set operator which is used to return the records that two SELECT statements have in common

• Generally used the same way as UNION above
• SELECT columns_names FROM table1 INTERSECT SELECT column_name FROM table2;

EXCEPT: set operator used to return all the records in the first SELECT statement that are not found in the second SELECT statement

• Generally used the same way as UNION above
• SELECT columns_names FROM table1 EXCEPT SELECT column_name FROM table2;

ANY|ALL: operator used to check subquery conditions used within a WHERE or HAVING clauses

• The ANY operator returns true if any subquery values meet the condition
• The ALL operator returns true if all subquery values meet the condition
• SELECT columns_names FROM table1 WHERE column_name operator (ANY|ALL) (SELECT column_name FROM table_name WHERE condition);

GROUP BY: statement often used with aggregate functions (COUNT, MAX, MIN, SUM, AVG) to group the result-set by one or more columns

• SELECT column_name1, COUNT(column_name2) FROM table_name WHERE condition GROUP BY column_name1 ORDER BY COUNT(column_name2) DESC;

HAVING: this clause was added to SQL because the WHERE keyword could not be used with aggregate functions

• SELECT COUNT(column_name1), column_name2 FROM table GROUP BY column_name2 HAVING COUNT(column_name1) > 5;

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Data Modification Queries

INSERT INTO: used to insert new records/rows in a table

• INSERT INTO table_name (column1, column2) VALUES (value1, value2);
• INSERT INTO table_name VALUES (value1, value2 …);

UPDATE: used to modify the existing records in a table

• UPDATE table_name SET column1 = value1, column2 = value2 WHERE condition;
• UPDATE table_name SET column_name = value;

DELETE: used to delete existing records/rows in a table

• DELETE FROM table_name WHERE condition;
• DELETE * FROM table_name;

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Reporting Queries

COUNT: returns the # of occurrences

• SELECT COUNT (DISTINCT column_name);

MIN() and MAX(): returns the smallest/largest value of the selected column

• SELECT MIN (column_names) FROM table_name WHERE condition;
• SELECT MAX (column_names) FROM table_name WHERE condition;

AVG(): returns the average value of a numeric column

• SELECT AVG (column_name) FROM table_name WHERE condition;

SUM(): returns the total sum of a numeric column

• SELECT SUM (column_name) FROM table_name WHERE condition;

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Join Queries

INNER JOIN: returns records that have matching value in both tables

• SELECT column_names FROM table1 INNER JOIN table2 ON table1.column_name=table2.column_name;
• SELECT table1.column_name1, table2.column_name2, table3.column_name3 FROM ((table1 INNER JOIN table2 ON relationship) INNER JOIN table3 ON relationship);

LEFT (OUTER) JOIN: returns all records from the left table (table1), and the matched records from the right table (table2)

• SELECT column_names FROM table1 LEFT JOIN table2 ON table1.column_name=table2.column_name;

RIGHT (OUTER) JOIN: returns all records from the right table (table2), and the matched records from the left table (table1)

• SELECT column_names FROM table1 RIGHT JOIN table2 ON table1.column_name=table2.column_name;

FULL (OUTER) JOIN: returns all records when there is a match in either left or right table

• SELECT column_names FROM table1 FULL OUTER JOIN table2 ON table1.column_name=table2.column_name;

Self JOIN: a regular join, but the table is joined with itself

• SELECT column_names FROM table1 T1, table1 T2 WHERE condition;

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View Queries

CREATE: create a view

• CREATE VIEW view_name AS SELECT column1, column2 FROM table_name WHERE condition;

DROP: drop a view

• DROP VIEW view_name;

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Altering Table Queries

ADD: add a column

• ALTER TABLE table_name ADD column_name column_definition;

MODIFY: change data type of column

• ALTER TABLE table_name MODIFY column_name column_type;

DROP: delete a column

• ALTER TABLE table_name DROP COLUMN column_name;

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Creating Table Query

CREATE: create a table

• CREATE TABLE table_name (
column1 datatype,
  column2 datatype,
  column3 datatype,
  column4 datatype,
  );