11-2. 데이터 불러오기

데이터 살펴보기

테이블에 있는 10개의 행만 출력하기
```
SELECT *
FROM `modulabs_project.data`
LIMIT 10
```
[결과 이미지를 넣어주세요]
전체 데이터는 몇 행으로 구성되어 있는지 확인하기
```
SELECT count(*)
FROM `modulabs_project.data`
```
[결과 이미지를 넣어주세요]

데이터 수 세기

COUNT 함수를 사용해서, 각 컬럼별 데이터 포인트의 수를 세어 보기

SELECT 
  count(InvoiceNo) COUNT_InvoiceNo,
  count(StockCode) COUNT_StockCode,
  count(Description)  COUNT_Description,
  count(Quantity) Count_Quantity, 
  count(InvoiceDate) Count_InvoiceDate,
  count(UnitPrice) Count_UnitPrice,
  count(CustomerID) Count_CustomerID,
  count(Country) Count_Country
FROM 
  `modulabs_project.data`

[결과 이미지를 넣어주세요]

11-4. 데이터 전처리 방법(1): 결측치 제거

컬럼 별 누락된 값의 비율 계산

각 컬럼 별 누락된 값의 비율을 계산

각 컬럼에 대해서 누락 값을 계산한 후, 계산된 누락 값을 UNION ALL을 통해 합치기

SELECT 
    column_name, 
		ROUND((total - column_value) / total * 100, 2) as missing_percentage
FROM
(
    SELECT 'InvoiceNo' AS column_name, COUNT(InvoiceNo) AS column_value, COUNT(*) AS total FROM `modulabs_project.data` UNION ALL
    SELECT 'StockCode' AS column_name, COUNT(StockCode) AS column_value, COUNT(*) AS total FROM `modulabs_project.data` UNION ALL
    SELECT 'Description' AS column_name, COUNT(Description) AS column_value, COUNT(*) AS total FROM `modulabs_project.data` UNION ALL
    SELECT 'Quantity' AS column_name, COUNT(Quantity) AS column_value, COUNT(*) AS total FROM `modulabs_project.data` UNION ALL
    SELECT 'InvoiceDate' AS column_name, COUNT(InvoiceDate) AS column_value, COUNT(*) AS total FROM `modulabs_project.data` UNION ALL
    SELECT 'UnitPrice' AS column_name, COUNT(UnitPrice) AS column_value, COUNT(*) AS total FROM `modulabs_project.data` UNION ALL
    SELECT 'CustomerID' AS column_name, COUNT(CustomerID) AS column_value, COUNT(*) AS total FROM `modulabs_project.data` UNION ALL
    SELECT 'Country' AS column_name, COUNT(Country) AS column_value, COUNT(*) AS total FROM `modulabs_project.data`
)

[결과 이미지를 넣어주세요]

결측치 처리 전략

StockCode = '85123A'의 Description을 추출하는 쿼리문을 작성하기
```
SELECT 
	DISTINCT Description
FROM  
	`modulabs_project.data`
WHERE 
	StockCode = '85123A'
```
[결과 이미지를 넣어주세요]

결측치 처리

DELETE 구문을 사용하며, WHERE 절을 통해 데이터를 제거할 조건을 제시
```
DELETE FROM `modulabs_project.data`
WHERE 
  Description IS NULL OR CustomerID IS NULL;
```
[결과 이미지를 넣어주세요]