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# Queries
## Aggregation
1. Get the maximum IQ in all students (Try without aggregation first).
```sql
SELECT
first_name, iq
FROM
students
ORDER BY iq DESC
LIMIT 1;
```
2. Get the maximum IQ in all students (With aggregation).
```sql
SELECT
MAX(IQ) AS 'IQ'
FROM
students;
```
3. Get the oldest batch from the batches table.
```sql
SELECT
MIN(start_date) AS 'start date'
FROM
batches;
```
4. Fetch the number of batches that start with the word `Jedi`.
```sql
SELECT
COUNT(id)
FROM
batches
WHERE
name LIKE 'Jedi%';
```
5. Get the average IQ of all students (Without using `AVG`)
```sql
SELECT
SUM(iq) / COUNT(iq) as 'Average IQ'
FROM
students;
```
6. Get the average IQ of students in all batches.
```sql
SELECT
AVG(IQ) AS 'IQ'
FROM
students
WHERE
batch_id IS NOT NULL;
```
7. Find the average IQ of students in each batch.
```sql
SELECT
batch_id, AVG(iq)
FROM
students
GROUP BY batch_id;
```
8. Find the total number of students in each batch.
```sql
SELECT
b.name, COUNT(s.id)
FROM
batches b
LEFT JOIN
students s ON b.id = s.batch_id
GROUP BY b.id;
```
9. Get the total number of batches taught by each instructor.
```sql
SELECT
i.first_name, COUNT(b.id)
FROM
instructors i
LEFT JOIN
batches b ON i.id = b.instructor_id
GROUP BY i.id;
```
10. Find the average IQ of students in batches with batch ID `1` and `2`.
```sql
SELECT
batch_id, AVG(iq)
FROM
students
WHERE batch_id IN (1, 2)
GROUP BY batch_id;
```
11. Find count of students that are part of batches that have average IQ greater than `120`.
```sql
SELECT
batch_id, AVG(iq) as avg_iq, COUNT(iq)
FROM
students
GROUP BY batch_id
HAVING avg_iq > 130;
```
## Built-in Functions
1. Get the average IQ of all the students rounded to the nearest integer.
```sql
SELECT
ROUND(AVG(iq))
FROM
students;
```
2. Find all batches whose name is longer than 10 characters.
```sql
SELECT
id, name
FROM
batches
WHERE
LENGTH(name) > 10;
```
3. Find all batches whose name's first 10 characters contains the string `sher`
```sql
SELECT
id, name
FROM
batches
WHERE
LOCATE('sher', SUBSTR(name, 1, 10)) > 0;
```
4. Get all batches that have started on a Sunday
```sql
SELECT
id, name
FROM
batches
WHERE
DAYNAME(start_date) = 'Sunday';
```
5. Get all batches that have been running for more than 10 years
```sql
SELECT
id, name, start_date
FROM
batches
WHERE
(DATEDIFF(NOW(), start_date) / 365) > 10;
```
6. Print the name and the instructor's id for each batch. If no instructor is assigned, print `NO INSTRUCTOR`.
```sql
SELECT
name, IFNULL(instructor_id, 'NO INSTRUCTOR')
FROM
batches;
```
7. Print the name and IQ of each student. If the IQ of the student is less than 100, print `LOW IQ` instead.
```sql
SELECT
first_name, last_name, IF(iq < 100, 'LOW IQ', iq)
FROM
students;
```
8. For each student print the name and their IQ category.
| IQ | Category |
| --------- | -------- |
| < 100 | LOW IQ |
| 100 - 150 | MEDIUM |
| > 150 | HIGH |
```sql
SELECT
first_name,
last_name,
CASE
WHEN iq > 150 THEN 'HIGH IQ'
WHEN iq BETWEEN 100 AND 150 THEN 'MEDIUM IQ'
ELSE 'LOW IQ'
END
FROM
students;
```

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# Queries
## Aggregation
1. Get the maximum IQ in all students (Try without aggregation first).
```sql
```
2. Get the maximum IQ in all students (With aggregation).
```sql
```
3. Get the oldest batch from the batches table.
```sql
```
4. Fetch the number of batches that start with the word `Jedi`.
```sql
```
5. Get the average IQ of all students (Without using `AVG`)
```sql
```
6. Get the average IQ of students in all batches.
```sql
```
7. Find the average IQ of students in each batch.
```sql
```
8. Find the total number of students in each batch.
```sql
```
9. Get the total number of batches taught by each instructor.
```sql
```
10. Find the average IQ of students in batches with batch ID `1` and `2`.
```sql
```
11. Find count of students that are part of batches that have average IQ greater than `120`.
```sql
```
## Built-in Functions
1. Get the average IQ of all the students rounded to the nearest integer.
```sql
```
2. Find all batches whose name is longer than 10 characters.
```sql
```
3. Find all batches whose name's first 10 characters contains the string `sher`
```sql
```
4. Get all batches that have started on a Sunday
```sql
```
5. Get all batches that have been running for more than 10 years
```sql
```
6. Print the name and the instructor's id for each batch. If no instructor is assigned, print `NO INSTRUCTOR`.
```sql
```
7. Print the name and IQ of each student. If the IQ of the student is less than 100, print `LOW IQ` instead.
```sql
```
8. For each student print the name and their IQ category.
| IQ | Category |
| --------- | -------- |
| < 100 | LOW IQ |
| 100 - 150 | MEDIUM |
| > 150 | HIGH |
```sql
```

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# Group by and built-in functions
- [Group by and built-in functions](#group-by-and-built-in-functions)
- [GROUP BY clause](#group-by-clause)
- [Problems](#problems)
- [MySQL functions](#mysql-functions)
- [Numeric functions](#numeric-functions)
- [String functions](#string-functions)
- [Date and time functions](#date-and-time-functions)
- [Miscellaneous functions](#miscellaneous-functions)
## GROUP BY clause
The GROUP BY clause groups a set of rows into a set of summary rows by values of columns or expressions. The GROUP BY clause returns one row for each group. In other words, it reduces the number of rows in the result set
**Keyword**: `GROUP BY`
**Syntax**: `SELECT column1, column2, ... FROM table_name GROUP BY column1, column2, ...`
In this syntax, you place the GROUP BY clause after the FROM and WHERE clauses. After the GROUP BY keywords, you place is a list of comma-separated columns or expressions to group rows.
> MySQL evaluates the GROUP BY clause after the FROM and WHERE clauses and before the HAVING, SELECT, DISTINCT, ORDER BY and LIMIT clauses
![Group By](https://www.mysqltutorial.org/wp-content/uploads/2021/07/MySQL-Group-By.svg)
Fields that are not encapsulated within an aggregate function and must be included in the GROUP BY Clause at the end of the SQL statement.
For example, the following query will return the number of students in each batch:
```sql
SELECT
batch_id, AVG(iq)
FROM
students
GROUP BY batch_id;
```
You can also use the `HAVING` clause to filter the result set.
For example, the following query will return the number of students in each batch where each batches average IQ is greater than or equal to 100:
```sql
SELECT
batch_id, AVG(iq)
FROM
students
GROUP BY batch_id
HAVING AVG(iq) >= 100;
```
### Problems
1. [Aggregation - I](https://leetcode.com/problems/duplicate-emails/)
2. [Aggregation - II](https://leetcode.com/problems/customer-placing-the-largest-number-of-orders/)
3. [Aggregation - III](https://leetcode.com/problems/classes-more-than-5-students/)
---
## MySQL functions
MySQL has a number of built-in functions that can be used to perform common tasks.
They are divided in the following categories:
1. Numeric
2. String
3. Date and time
4. Miscellaneous
## Numeric functions
| Function | Description | Example |
| -------- | --------------------------------- | ------------------------ |
| ABS | Absolute value | SELECT ABS(-1) |
| CEIL | Round up to the nearest integer | SELECT CEIL(1.5) |
| FLOOR | Round down to the nearest integer | SELECT FLOOR(1.5) |
| ROUND | Round to the given precision | SELECT ROUND(1.54, 1) |
| TRUNCATE | Truncate to the given precision | SELECT TRUNCATE(1.54, 1) |
| RAND | Generate a random number | SELECT RAND() |
See more function [here](https://dev.mysql.com/doc/refman/8.0/en/numeric-functions.html).
## String functions
| Function | Description | Example |
| -------- | -------------------------------- | --------------------------------------------- |
| LENGTH | Length of the string | SELECT LENGTH('Kattapa') |
| LOWER | Convert to lowercase | SELECT LOWER('Kattapa') |
| UPPER | Convert to uppercase | SELECT UPPER('Kattapa') |
| LTRIM | Trim leading spaces | SELECT LTRIM(' Kattapa') |
| RTRIM | Trim trailing spaces | SELECT RTRIM('Kattapa ') |
| TRIM | Trim leading and trailing spaces | SELECT TRIM(' Kattapa ') |
| SUBSTR | Extract a substring | SELECT SUBSTR('Namma Bengaluru', 1, 3) |
| LEFT | Extract left substring | SELECT LEFT('Namma Bengaluru', 3) |
| RIGHT | Extract right substring | SELECT RIGHT('Namma Bengaluru', 3) |
| LOCATE | Find the position of a substring | SELECT LOCATE('Bengaluru', 'Namma Bengaluru') |
See more function [here](https://dev.mysql.com/doc/refman/8.0/en/string-functions.html).
## Date and time functions
| Function | Description | Example |
| --------- | ---------------------------- | --------------------------------------------- |
| NOW | Current date and time | SELECT NOW() |
| CURDATE | Current date | SELECT CURDATE() |
| CURTIME | Current time | SELECT CURTIME() |
| YEAR | Year of the date | SELECT YEAR('2020-01-01') |
| MONTH | Month of the date | SELECT MONTH('2020-01-01') |
| DAY | Day of the date | SELECT DAY('2020-01-01') |
| DAYNAME | Day of the week | SELECT DAYNAME('2020-01-01') |
| DAYOFWEEK | Day of the week | SELECT DAYOFWEEK('2020-01-01') |
| DATE_ADD | Add a date | SELECT DATE_ADD('2020-01-01', INTERVAL 1 DAY) |
| DATE_SUB | Subtract a date | SELECT DATE_SUB('2020-01-01', INTERVAL 1 DAY) |
| DATEDIFF | Difference between two dates | SELECT DATEDIFF('2020-01-01', '2020-01-02') |
## Miscellaneous functions
| Function | Description | Example |
| -------- | ------------------------------- | ------------------------------------------------------ |
| IFNULL | Replace NULL values | SELECT IFNULL(batch_id, 'NO BATCH') |
| COALESCE | Replace NULL values recursively | SELECT COALESCE(batch_id, phone, email, first_name) |
| IF | Conditional expression | SELECT IF(batch_id = 1, 'YES', 'NO') |
| CASE | Conditional expression | SELECT CASE WHEN batch_id = 1 THEN 'YES' ELSE 'NO' END |