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1. EXISTS

EXISTS ×ÓÅÌÎÊ¿ÉÓÃÓÚ¼ì²éÖµÊÇ·ñ±£´æÓÚÁíÒ»¸ö±í»òÅÌÎÊÖÐ ¡£Óë IN Ïà±È£¬EXISTS ¸üÓÐÀûÓÚÖ´ÐÐÍýÏëµÄÓÅ»¯ ¡£

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SELECT *
FROM customers
WHERE EXISTS (SELECT *
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               WHERE orders.customer_id = customers.customer_id)

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2. ×ÓÅÌÎÊ

×ÓÅÌÎÊ¿ÉÒÔÓë½ÏÁ¿ÔËËã·ûÒ»ÆðʹÓã¬ÀýÈç = »ò !=£¬ÒÔ¼ì²éÖµÊÇ·ñÓë×ÓÅÌÎÊ·µ»ØµÄֵƥÅä ¡£×ÓÅÌÎÊÔÊÐíÎÞаµØÖ¸¶¨ÖØ´óɸѡÌõ¼þ ¡£

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SELECT *
FROM customers
WHERE customer_id = (SELECT customer_id
                       FROM orders
                       WHERE product_id = 1)

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3. JOIN

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SELECT *
FROM customers
INNER JOIN orders ON customers.customer_id = orders.customer_id
WHERE orders.product_id = 1

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4. UNION

UNION ²Ù×÷·û¿ÉÒÔ½«¶à¸ö±í»òÅÌÎʵÄЧ¹ûºÏ²¢µ½Ò»¸ö±íÖÐ ¡£Í¨¹ýʹÓà UNION ºÍ DISTINCT£¬¿ÉÒÔʵÏÖÀàËÆÓÚ IN µÄ¹¦Ð§ ¡£

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SELECT DISTINCT customer_id
FROM (SELECT customer_id FROM orders
      UNION
      SELECT customer_id FROM customers)

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