完美解析SQL只需要简单的十个步骤

SELECT A.x + A.y AS z
FROM A
WHERE z = 10 -- z 在此处不可用，因为SELECT是最后执行的语句！

SELECT A.x + A.y AS z
FROM AWHERE (A.x + A.y) = 10

<table reference> ::=    <table name>
  | <derived table>
  | <joined table>

FROM a, b, c, d, e, f, g, h
WHERE a.a1 = b.bxAND a.a2 = c.c1AND d.d1 = b.bc
-- etc...

-- This table reference contains authors and their books.
-- There is one record for each book and its author.
-- authors without books are NOT included
author JOIN book ON author.id = book.author_id

-- This table reference contains authors and their books
-- There is one record for each book and its author.
-- ... OR there is an "empty" record for authors without books
-- ("empty" meaning that all book columns are NULL)
author LEFT OUTER JOIN book ON author.id = book.author_id

-- Using IN
FROM author
WHERE author.id IN (SELECT book.author_id FROM book)

-- Using EXISTS
FROM author
WHERE EXISTS (SELECT 1 FROM book WHERE book.author_id = author.id)

-- Find only those authors who also have books
SELECT DISTINCT first_name, last_name
FROM author
JOIN book ON author.id = book.author_id

-- Using IN
FROM author
WHERE author.id NOT IN (SELECT book.author_id FROM book)

-- Using EXISTS
FROM author
WHERE NOT EXISTS (SELECT 1 FROM book WHERE book.author_id = author.id)

author CROSS JOIN book

-- A derived table
FROM (SELECT * FROM author)

-- A derived table with an aliasFROM (SELECT * FROM author) a

-- Get authors' first and last names, and their age in days
SELECT first_name, last_name, age
FROM (
  SELECT first_name, last_name, current_date - date_of_birth age
  FROM author
)
-- If the age is greater than 10000 days
WHERE age > 10000

WITH a AS (
  SELECT first_name, last_name, current_date - date_of_birth age
  FROM author
)
SELECT *
FROM a
WHERE age > 10000

SELECT A.x, A.y, SUM(A.z)FROM AGROUP BY A.x, A.y