# 資料庫語法

## DISTINCT
用於返回單一值

SELECT DISTINCT column1, column2, ...
FROM table_name;

## ORDER BY
用於按升序或降序對結果集進行排序。
- ASC
預設值，ascending order 升序
- DESC
descending order 降序

SELECT column1, column2, ...
FROM table_name
ORDER BY column1, column2, ... ASC|DESC;

## NULL

-- IS NULL
SELECT column_names
FROM table_name
WHERE column_name IS NULL;

-- IS NOT NULL
SELECT column_names
FROM table_name
WHERE column_name IS NOT NULL;

## SELECT TOP
用於指定要返回的記錄數。

SELECT TOP number|percent column_name(s)
FROM table_name
WHERE condition;

> Not all database systems support the `SELECT TOP` clause. MySQL supports the `LIMIT` clause to select a limited number of records.

## MIN, MAX
MIN() 函數返回所選列的最小值，MAX() 函數返回所選列的最大值。

SELECT MIN(column_name), MAX(column_name)
FROM table_name
WHERE condition;

## COUNT, AVG, SUM
COUNT() 函數返回與指定條件匹配的行數。

SELECT COUNT(column_name)
FROM table_name
WHERE condition;

AVG() 函數返回數字列的平均值。

SELECT AVG(column_name)
FROM table_name
WHERE condition;

SUM() 函數返回數字列的總和。

SELECT SUM(column_name)
FROM table_name
WHERE condition;

## LIKE
`LIKE` 運算符在 `WHERE` 子句中用於搜索列中的指定模式。
百分號 (`%`) 代表零、一個或多個字符
下劃線 (`_`) 代表一個，單個字符

SELECT column1, column2, ...
FROM table_name
WHERE columnN LIKE pattern;

## IN
IN 運算符允許在 WHERE 子句中指定多個值。

SELECT column_name(s)
FROM table_name
WHERE column_name IN (value1, value2, ...);

SELECT column_name(s)
FROM table_name
WHERE column_name IN (SELECT STATEMENT);

## JOIN