LEFT JOIN 或 RIGHT JOIN 在GROUP BY 下如何利用COUNT统计有NULL的行
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LEFT JOIN :
SELECT * FROM DEPT D LEFT JOIN EMP E ON D.ID=E.DEPT_ID
RIGHT JOIN :
SELECT * FROM DEPT D RIGHT JOIN EMP E ON D.ID=E.DEPT_ID
INNER JOIN :
SELECT * FROM DEPT D INNER JOIN EMP E ON D.ID=E.DEPT_ID
JOIN : == INNER JOIN
SELECT * FROM DEPT D JOIN EMP E ON D.ID=E.DEPT_ID;
join + group by + count 的使用 : 按分组统计合并的组内记录数
SELECT *,COUNT(D.ID) NUM
FROM DEPT D LEFT JOIN EMP E ON D.ID=E.DEPT_ID
SELECT *,COUNT(D.ID) NUM
FROM DEPT D LEFT JOIN EMP E ON D.ID=E.DEPT_ID
GROUP BY D.`NAME`
SELECT *,COUNT(E.ID) NUM
FROM DEPT D LEFT JOIN EMP E ON D.ID=E.DEPT_ID
GROUP BY D.`NAME`
SELECT *,COUNT(E.ID) NUM
FROM DEPT D LEFT JOIN EMP E ON D.ID=E.DEPT_ID
GROUP BY D.`NAME`
总结:
count(A)函数的使用,A的利用很重要,看A列的目标记录行是否为NULL来判断是置 0 或 1
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