SQL 語法參考

45個條目

Basic SELECTSELECT

SELECT col1, col2 FROM table;

SELECT name, email FROM users;

ℹ Use * to select all columns (avoid in production)

SELECT with aliasSELECT

SELECT col AS alias FROM table;

SELECT first_name AS name, created_at AS joined FROM users;

SELECT DISTINCTSELECT

SELECT DISTINCT col FROM table;

SELECT DISTINCT country FROM customers;

WHERE clauseSELECT

SELECT ... FROM table WHERE condition;

SELECT * FROM orders WHERE status = 'pending' AND total > 100;

ORDER BYSELECT

SELECT ... ORDER BY col ASC|DESC;

SELECT name, price FROM products ORDER BY price DESC;

LIMIT / OFFSETSELECT

SELECT ... LIMIT n OFFSET m;

SELECT * FROM posts LIMIT 20 OFFSET 40;

ℹ SQL Server: use TOP n or FETCH NEXT

LIKE pattern matchSELECT

WHERE col LIKE 'pattern'

SELECT * FROM users WHERE email LIKE '%@gmail.com';

IN listSELECT

WHERE col IN (v1, v2, ...)

SELECT * FROM orders WHERE status IN ('pending', 'processing');

BETWEEN rangeSELECT

WHERE col BETWEEN v1 AND v2

SELECT * FROM orders WHERE total BETWEEN 100 AND 500;

IS NULL checkSELECT

WHERE col IS NULL | IS NOT NULL

SELECT * FROM users WHERE deleted_at IS NULL;

CASE expressionSELECT

CASE WHEN cond THEN val ... ELSE val END

SELECT name, CASE WHEN age >= 18 THEN 'adult' ELSE 'minor' END AS type FROM users;

COUNTAggregates

SELECT COUNT(*) | COUNT(col) FROM table;

SELECT COUNT(*) AS total, COUNT(email) AS with_email FROM users;

SUM / AVG / MIN / MAXAggregates

SELECT SUM(col), AVG(col), MIN(col), MAX(col) FROM table;

SELECT SUM(total) AS revenue, AVG(total) AS avg_order FROM orders;

GROUP BYAggregates

SELECT col, AGG(x) FROM table GROUP BY col;

SELECT country, COUNT(*) AS users FROM users GROUP BY country;

HAVINGAggregates

... GROUP BY col HAVING AGG(x) > n

SELECT country, COUNT(*) as cnt FROM users GROUP BY country HAVING COUNT(*) > 100;

GROUP BY with ORDERAggregates

... GROUP BY col ORDER BY AGG(x) DESC

SELECT status, COUNT(*) cnt FROM orders GROUP BY status ORDER BY cnt DESC;

INNER JOINJOINs

SELECT ... FROM a INNER JOIN b ON a.id = b.a_id;

SELECT u.name, o.total FROM users u INNER JOIN orders o ON u.id = o.user_id;

LEFT JOINJOINs

SELECT ... FROM a LEFT JOIN b ON a.id = b.a_id;

SELECT u.name, COUNT(o.id) FROM users u LEFT JOIN orders o ON u.id = o.user_id GROUP BY u.id;

RIGHT JOINJOINs

SELECT ... FROM a RIGHT JOIN b ON a.id = b.a_id;

SELECT o.id, u.name FROM orders o RIGHT JOIN users u ON o.user_id = u.id;

FULL OUTER JOINJOINs

SELECT ... FROM a FULL OUTER JOIN b ON condition;

SELECT a.id, b.id FROM table_a a FULL OUTER JOIN table_b b ON a.key = b.key;

ℹ Not supported in MySQL 8 — use UNION of LEFT + RIGHT JOIN

SELF JOINJOINs

SELECT ... FROM t AS a JOIN t AS b ON a.col = b.col;

SELECT e.name, m.name AS manager FROM employees e LEFT JOIN employees m ON e.manager_id = m.id;

CROSS JOINJOINs

SELECT ... FROM a CROSS JOIN b;

SELECT colors.name, sizes.label FROM colors CROSS JOIN sizes;

ℹ Returns Cartesian product — rows(a) × rows(b)

Subquery in WHERESubqueries

WHERE id IN (SELECT id FROM ...)

SELECT * FROM products WHERE category_id IN (SELECT id FROM categories WHERE active = 1);

Scalar subquerySubqueries

SELECT (SELECT COUNT(*) FROM ...) AS n

SELECT name, (SELECT COUNT(*) FROM orders WHERE user_id = u.id) AS order_count FROM users u;

EXISTSSubqueries

WHERE EXISTS (SELECT 1 FROM ...)

SELECT * FROM users u WHERE EXISTS (SELECT 1 FROM orders o WHERE o.user_id = u.id);

CTE (WITH clause)Subqueries

WITH cte AS (SELECT ...) SELECT * FROM cte;

WITH recent AS (SELECT * FROM orders WHERE created_at > NOW() - INTERVAL '7 days') SELECT user_id, COUNT(*) FROM recent GROUP BY user_id;

Recursive CTESubqueries

WITH RECURSIVE cte AS (base UNION ALL recursive) ...

WITH RECURSIVE nums(n) AS (SELECT 1 UNION ALL SELECT n+1 FROM nums WHERE n < 10) SELECT n FROM nums;

ROW_NUMBER()Window Fns

ROW_NUMBER() OVER (PARTITION BY ... ORDER BY ...)

SELECT name, ROW_NUMBER() OVER (PARTITION BY dept ORDER BY salary DESC) AS rank FROM employees;

RANK / DENSE_RANKWindow Fns

RANK() OVER (ORDER BY col DESC)

SELECT name, salary, RANK() OVER (ORDER BY salary DESC) AS pay_rank FROM employees;

ℹ RANK skips numbers on ties; DENSE_RANK does not

LAG / LEADWindow Fns

LAG(col, n) OVER (ORDER BY ...)

SELECT date, revenue, LAG(revenue,1) OVER (ORDER BY date) AS prev_revenue FROM daily_sales;

SUM() OVER (running total)Window Fns

SUM(col) OVER (ORDER BY col ROWS UNBOUNDED PRECEDING)

SELECT date, amount, SUM(amount) OVER (ORDER BY date) AS running_total FROM transactions;

NTILE(n)Window Fns

NTILE(n) OVER (ORDER BY col)

SELECT name, salary, NTILE(4) OVER (ORDER BY salary) AS quartile FROM employees;

INSERT single rowDML

INSERT INTO table (col1, col2) VALUES (v1, v2);

INSERT INTO users (name, email) VALUES ('Alice', '[email protected]');

INSERT multiple rowsDML

INSERT INTO table (cols) VALUES (r1), (r2), ...;

INSERT INTO tags (name) VALUES ('sql'), ('database'), ('tutorial');

INSERT … SELECTDML

INSERT INTO t1 (cols) SELECT cols FROM t2;

INSERT INTO archive SELECT * FROM orders WHERE created_at < '2023-01-01';

UPDATEDML

UPDATE table SET col = val WHERE condition;

UPDATE users SET status = 'inactive', updated_at = NOW() WHERE last_login < NOW() - INTERVAL '90 days';

DELETEDML

DELETE FROM table WHERE condition;

DELETE FROM sessions WHERE expires_at < NOW();

ℹ Always include WHERE — omitting it deletes all rows

UPSERT (ON CONFLICT)DML

INSERT … ON CONFLICT (col) DO UPDATE SET …

INSERT INTO settings (key, value) VALUES ('theme', 'dark') ON CONFLICT (key) DO UPDATE SET value = EXCLUDED.value;

ℹ PostgreSQL syntax. MySQL uses ON DUPLICATE KEY UPDATE.

TRUNCATEDML

TRUNCATE TABLE table;

TRUNCATE TABLE temp_imports;

ℹ Faster than DELETE; resets auto-increment; cannot be rolled back in some DBs

CREATE TABLEDDL

CREATE TABLE name (col type constraints, ...);

CREATE TABLE users (id SERIAL PRIMARY KEY, email VARCHAR(255) UNIQUE NOT NULL, created_at TIMESTAMPTZ DEFAULT NOW());

ALTER TABLE ADD COLUMNDDL

ALTER TABLE t ADD COLUMN col type;

ALTER TABLE users ADD COLUMN phone VARCHAR(20);

ALTER TABLE RENAMEDDL

ALTER TABLE t RENAME COLUMN old TO new;

ALTER TABLE users RENAME COLUMN username TO display_name;

CREATE INDEXDDL

CREATE INDEX name ON table (col);

CREATE INDEX idx_users_email ON users (email);
CREATE UNIQUE INDEX idx_users_email_unique ON users (email);

DROP TABLEDDL

DROP TABLE [IF EXISTS] table;

DROP TABLE IF EXISTS temp_staging;

FOREIGN KEYDDL

REFERENCES other_table(col) ON DELETE action

CREATE TABLE orders (id SERIAL PRIMARY KEY, user_id INT REFERENCES users(id) ON DELETE CASCADE);

SQL子句執行順序

FROM→

JOIN→

WHERE→

GROUP BY→

HAVING→

SELECT→

DISTINCT→

ORDER BY→

LIMIT

這是邏輯的處理順序。WHERE在SELECT之前執行，因此無法在以下位置引用SELECT別名 WHERE子句 — 改用CTE或子查詢。HAVING可以過濾 GROUP BY聚合上。

NULL處理規則

表達式	結果	備註
NULL = NULL	FALSE / UNKNOWN	Use IS NULL instead
NULL != NULL	UNKNOWN	Any comparison with NULL is UNKNOWN
NULL + 5	NULL	Arithmetic on NULL propagates NULL
COALESCE(NULL, 'x')	'x'	Returns first non-NULL value
NULLIF(a, a)	NULL	Returns NULL if both args are equal
COUNT(*)	Counts all rows	COUNT(col) skips NULLs

SQL快速參考常見問題

SQL子句的執行順序是什麼？

SQL子句按以下邏輯順序執行：FROM → JOIN → WHERE → GROUP BY → HAVING → SELECT → DISTINCT → ORDER BY → LIMIT/OFFSET。這很重要，因為WHERE不能引用SELECT別名，但HAVING可以引用GROUP BY欄位。

INNER JOIN和LEFT JOIN有什麼區別？

INNER JOIN僅返回兩個表中連接條件匹配的行。LEFT JOIN（LEFT OUTER JOIN）返回左表中的所有行，對不匹配的右表欄位返回NULL。當您需要左表中的所有記錄，無論右側是否有匹配時，使用LEFT JOIN。

SQL中的視窗函數是什麼？

視窗函數在與當前行相關的一組行中計算值，而不像GROUP BY聚合那樣將其折疊為單行。常見的視窗函數：ROW_NUMBER(), RANK(), DENSE_RANK(), LAG(), LEAD(), SUM() OVER(), AVG() OVER()。它們使用OVER()子句來定義視窗。

什麼是CTE（公共資料表運算式）？

CTE是在SELECT, INSERT, UPDATE或DELETE語句之前使用WITH關鍵字定義的命名臨時結果集。CTE比巢狀子查詢更易讀，還可以是遞迴的（WITH RECURSIVE），用於組織架構或類別樹等層次資料。

Docker 速查表 — 命令參考 Docker Compose 轉換器 Git 速查表 — Git 命令參考 Bash 速查表 — 命令參考 & 範例 Chmod 計算器 — Unix 檔案權限計算器 Cron 表達式產生器 — 排程產生器

NULL處理規則

表達式	結果	備註
NULL = NULL	FALSE / UNKNOWN	Use IS NULL instead
NULL != NULL	UNKNOWN	Any comparison with NULL is UNKNOWN
NULL + 5	NULL	Arithmetic on NULL propagates NULL
COALESCE(NULL, 'x')	'x'	Returns first non-NULL value
NULLIF(a, a)	NULL	Returns NULL if both args are equal
COUNT(*)	Counts all rows	COUNT(col) skips NULLs

SQL 語法 參考

SQL子句執行順序

NULL處理規則

SQL快速參考常見問題

SQL子句的執行順序是什麼？

INNER JOIN和LEFT JOIN有什麼區別？

SQL中的視窗函數是什麼？

什麼是CTE（公共資料表運算式）？

相關工具

SQL 語法 參考

SQL子句執行順序

NULL處理規則

SQL快速參考常見問題

SQL子句的執行順序是什麼？

INNER JOIN和LEFT JOIN有什麼區別？

SQL中的視窗函數是什麼？

什麼是CTE（公共資料表運算式）？

相關工具

SQL 語法參考

SQL 語法參考