1. 下面哪些语句会使用到索引，哪些索引起了作用？

CREATE TABLE t1 (    id int unsigned NOT NULL auto_increment,    a int unsigned NOT NULL DEFAULT 0,    b int unsigned NOT NULL DEFAULT 0,    c int unsigned NOT NULL DEFAULT 0,    PRIMARY KEY(id),    KEY abc (a, b, c) USING BTREE);(1). SELECT * FROM t1 WHERE a=3 AND b=5 AND c=4;(2). SELECT * FROM t1 WHERE c=4 AND b=6 AND a=3;(3). SELECT * FROM t1 WHERE a=3 AND c=7;(4). SELECT * FROM t1 WHERE a=3 AND b>7 AND c=3;(5). SELECT * FROM t1 WHERE b=3 AND c=4;(6). SELECT * FROM t1 WHERE a>4 AND b=7 AND c=9;(7). SELECT * FROM t1 WHERE a=3 ORDER BY b;(8). SELECT * FROM t1 WHERE a=3 ORDER BY c;(9). SELECT * FROM t1 WHERE b=3 ORDER BY a;复制代码

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