问题中的这两种SQL哪个效率高?

Question

SELECT T3.ID,T3.NAME
FROM ZY_UNIT_WARD T1 
LEFT JOIN XT_UNIT T2 ON T1.UNITID=T2.ID
LEFT JOIN zy_ward T3 ON T2.ID=T3.UNITID
WHERE T2.ID=8

SELECT T1.ID,T1.NAME FROM zy_ward T1,XT_UNIT T2,ZY_UNIT_WARD T3
WHERE T1.ID=T3.WARDID AND T2.ID=T3.UNITID

Answer 1

前者效率高，后者是取笛卡儿积，相当于二重循环，复杂度N^2，前者是hash匹配，复杂度LogN