现需要统计表tab1中，col1列中值为空的行数，以下写法，哪些是正确的：--CDE A select count(*) from tab1 where col1=null; B select count(col1) from tab1 where col1 is null; C select count(*) from tab1 where col1 is null; D select count(nvl(col1,’Y’)) from tabl1 where nvl(col1,’Y’)=’Y’; E select count(nvl(col1,2)) from tabl1 where nvl(col1,2)=2; F select count(col1) from tab1 where col1 = null; Oracle中需要对DEMO用户下的表TAB1及其上的索引收集统计信息，采样比例为10%，并且收集完成后，立即失效相关SQL的游标，以下哪个命令可以达成上述目的？--A A exec dbms_stats.gather_table_stats('DEMO','TAB1',estimate_percent=>10,cascade=>true,no_invalidate=>false); B exec dbms_stats.gather_table_stats('DEMO','TAB1',estimate_percent=>10%,cascade=>true,no_invalidate=>ture); C exec dbms_stats.gather_table_stats('DEMO','TAB1',estimate_percent=>10,cascade=>true,no_invalidate=>true); D exec dbms_stats.gather_table_stats('DEMO','TAB1',estimate_percent=>10,cascade=>false,no_invalidate=>false); E exec dbms_stats.gather_table_stats('DEMO','TAB1',estimate_percent=>0.1,cascade=>true,no_invalidate=>false);