Mysql关于join on where(and)的区别
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直接看实例;
test表
test2表
JOIN
SELECT * FROM test JOIN test2 ON test.`book_id` =test2.`book_id` WHERE test.book_name='java'
与
SELECT * FROM test JOIN test2 ON test.`book_id` =test2.`book_id` AND test.book_name='java'
的结果是一样的。
LEFT JOIN
SELECT * FROM test LEFT JOIN test2 ON test.`book_id` =test2.`book_id` WHERE test.book_name='java'
SELECT * FROM test LEFT JOIN test2 ON test.`book_id` =test2.`book_id` AND test.book_name='java'
RIGHT JOIN
SELECT * FROM thai_test RIGHT JOIN thai_test2 ON thai_test.`book_id` =thai_test2.`book_id` WHERE thai_test.book_name='java'
SELECT * FROM thai_test RIGHT JOIN thai_test2 ON thai_test.`book_id` =thai_test2.`book_id` AND thai_test.book_name='java'
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ps.以下为转载
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1,where 是在两个表join完成后,再附上where条件。
2,而 and 则是在表连接前过滤A表或B表里面哪些记录符合连接条件,同时会兼顾是left join还是right join。即假如是左连接的话,如果左边表的某条记录不符合连接条件,那么它不进行连接,但是仍然留在结果集中(此时右边部分的连接结果为NULL)。
3,建议尽量用where来过滤条件,以避免复杂的逻辑考虑。(除非在某些情况下(后接其他sql语句),用and会报错,才用and,但要考虑是否影响正确结果。)
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