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单表查询-having和where区别

news 2025/9/12 5:43:53

having和where的区别

差异之处：
Having：用于分组后过滤数据，可以使用聚合函数
Where：用于分组前过滤数据，不可使用聚合函数
相同之处：
都是用于过滤数据的。

案例分析

语句执行顺序：from->where->group by ->having->select->order
从上面的执行顺序可以知道where优先级比having高。

例子1

SQL> select count(*),fid from t1 group by fid having fid=1;

LINEID COUNT(*) FID
---------- -------------------- -----------
1 166 1

1 #NSET2: [510, 250->1, 4]
2 #PRJT2: [510, 250->1, 4]; exp_num(2), is_atom(FALSE)
3 #SLCT2: [510, 250->1, 4]; T1.FID = 1
4 #SAGR2: [509, 10000->10000, 4]; grp_num(1), sfun_num(1), opt_num(0), distinct_flag[0]; slave_empty(0) keys(T1.FID)
5 #PARALLEL: [317, 3000000->3000000, 4]; scan_type(FULL)
6 #SSCN: [317, 3000000->3000000, 4]; IDX_DM_T1_FID(T1); btr_scan(1)

Statistics
-----------------------------------------------------------------
0   data pages changed
0   undo pages changed
2440   logical reads
0   physical reads
0   redo size
197   bytes sent to client
119   bytes received from client
1   roundtrips to/from client
0   sorts (memory)
0   sorts (disk)
1   rows processed
0   io wait time(ms)
38   exec time(ms)

used time: 38.918(ms). Execute id is 703.

例子2

SQL> select count(*),fid from t1 where fid=1 group by fid;

LINEID COUNT(*) FID
---------- -------------------- -----------
1 166 1

1 #NSET2: [1, 1->1, 4]
2 #PRJT2: [1, 1->1, 4]; exp_num(2), is_atom(FALSE)
3 #SLCT2: [1, 1->1, 4]; exp_sfun9 > 0
4 #AAGR2: [1, 1->1, 4]; grp_num(0), sfun_num(1), distinct_flag[0]; slave_empty(0)
5 #PARALLEL: [1, 166->166, 4]; scan_type(FULL)
6 #SSEK2: [1, 166->166, 4]; scan_type(ASC), IDX_DM_T1_FID(T1), scan_range[1,1]

Statistics
-----------------------------------------------------------------
0   data pages changed
0   undo pages changed
136   logical reads
0   physical reads
0   redo size
190   bytes sent to client
118   bytes received from client
1   roundtrips to/from client
0   sorts (memory)
0   sorts (disk)
1   rows processed
0   io wait time(ms)
1   exec time(ms)

used time: 1.606(ms). Execute id is 704.

上面例子的结果都是一样的，但是执行效率是where更快。
过滤数据上，前面也提到where优先级高于having，也就是说where先过滤数据再分组，having是先分组再过滤数据。