Oracle - SELECT 1 FROM table; SELECT COUNT(1) FROM table;

SELECT 1

SELECT 1 FROM table
SELECT anyColumn FROM table
SELECT * FROM table

效率上：1 > anyColumn > * 因为 SELECT 1 不用查字典。

SELECT 1, r.mc FROM ry r WHERE ROWNUM <=10

Oracle - SELECT 1 FROM table; SELECT COUNT(1) FROM table;

SELECT 1 给查询的记录加一个临时列。查看是否有记录，一般作为查询条件使用。

COUNT()

SELECT COUNT(*) FROM ry WHERE ROWNUM <= 10
SELECT COUNT(1) FROM ry WHERE ROWNUM <= 10
SELECT COUNT(13) FROM ry WHERE ROWNUM <= 10
SELECT COUNT('!@#') FROM ry WHERE ROWNUM <= 10

Oracle - SELECT 1 FROM table; SELECT COUNT(1) FROM table;

COUNT(1) 其中 1 是常量（也可以为其他整型常量（统计上方便））