什么是SQL？

SQL（Structured Query Language）用于訪問和操作數據庫的結構化查詢語言。

數據庫包含一個或多個表，每個表均有名稱標識，包含數據的記錄（行）。

典型的SQL語句

1. SELEC語句

SELECT DISTINCT name
FROM accounts
WHERE mobile IS NOT NULLAND is_activated = TRUE
ORDER BY created_at;SELECT *
FROM roles
WHERE project_id = -1
LIMIT 4;-- 統計計算
SELECT count(id), sum(id), avg(id), max(project_id), min(project_id) project_id
FROM products
WHERE name LIKE '%o' AND name LIKE 'n%'
group by project_id;SELECT *
FROM organizations_editions
WHERE edition_id BETWEEN 1 AND 7AND state in (2, 6);-- having與group by聯用，不能和where共存
SELECT project_id, string_agg(text(name), ', ')
FROM products
GROUP BY project_id
HAVING count(1) > 3;-- exists語句
SELECT *
FROM products
WHERE EXISTS(SELECT * FROM projects WHERE projects.disabled = TRUE AND products.disabled = FALSE);-- ALL && ANY(ANY任意匹配，有返回值；ALL需要全匹配，無返回)
SELECT *
FROM editions_modules
WHERE module_id = ANY (SELECT id FROM modules WHERE key = 'overview');SELECT *
FROM editions_modules
WHERE module_id = ALL (SELECT id FROM modules WHERE key = 'overview');-- SELECT INTO復制一個表old_table，到新數據庫new_db的新new_table表
SELECT *
INTO new_table IN new_db
FROM old_table
WHERE condition;

2. INSERT INTO語句

INSERT INTO users_roles(user_id, role_id, enable)
VALUE(1, 1, TRUE)

3. UPDATE語句

UPDATE users
SET disabled = TRUE
WHERE email is NULL AND mobile IS NOT NULL;

4. DELETE語句

DELETE FROM users 
WHERE name IS NULL;

5. JOIN / INNER JOIN / LEFT JOIN / RIGHT JOIN / FULL JOIN / SELF JOIN