left join 连接查询的where是全部连接完之后判断还是连接前判断

在使用 LEFT JOIN 连接查询时，WHERE 子句是在连接完成之后进行筛选的。这意味着，首先会将两个表连接起来，然后再过滤掉不符合 WHERE 条件的行。因此，LEFT JOIN 会返回左表中所有的行，而右表中没有匹配的行则返回 NULL。如果你需要在连接之前进行筛选，可以使用子查询或者临时表来实现。

相关问题

where连接和left join连接的区别

WHERE连接和LEFT JOIN连接的区别在于它们执行连接操作的方式不同。

WHERE连接是通过在WHERE子句中指定连接条件来连接两个或多个表。它只返回在连接条件为真的行，并且不会返回NULL值。

LEFT JOIN连接是通过在LEFT JOIN子句中指定连接条件来连接两个或多个表。它返回左表中的所有行和右表中满足连接条件的行，并在右表中找不到匹配行的地方返回NULL值。

换句话说，WHERE连接只返回匹配的行，而LEFT JOIN连接返回左表中的所有行和右表中匹配的行，如果右表中没有匹配的行，则返回NULL。

将他们进行left join连接

将上述引用中的表进行left join连接的SQL语句如下所示：

SELECT 
    a.login_date, 
    log_cnt, 
    yd_date, 
    jk_date, 
    xd_date, 
    fk_date
FROM 
    (SELECT 
        login_date, 
        COUNT(DISTINCT customer_id) AS log_cnt
    FROM 
        jfu_dwddwd_user_wk_login_di
    WHERE 
        login_date BETWEEN '20231101' AND '20231110'
        AND p_da) a
LEFT JOIN 
    (SELECT 
        loan_date, 
        COUNT(DISTINCT customer_id) AS yd_date
    FROM 
        bd_srcwk_t_order_detail_log
    WHERE 
        loan_date BETWEEN '20231101 000000' AND '20231110 000000'
        AND) b ON a.login_date = b.loan_date
LEFT JOIN 
    (SELECT 
        login_date, 
        COUNT(DISTINCT customer_id) AS jk_date
    FROM 
        bd_srcwk_t_distribute_log
    WHERE 
        create_time BETWEEN p_day_id AND y_id
    GROUP BY 
        login_date) c ON a.login_date = c.login_date
LEFT JOIN 
    (SELECT 
        loan_date, 
        COUNT(DISTINCT customer_id) AS xd_date
    FROM 
        bd_srcwk_wk_loan_order
    WHERE 
        loan_date BETWEEN '20231101 000000' AND '20231110 000000'
        AND p_day_id = date_sub(getdate(), 1)
    GROUP BY 
        loan_date) d ON a.login_date = d.loan_date
LEFT JOIN 
    (SELECT 
        loan_time, 
        COUNT(DISTINCT customer_id) AS fk_date
    FROM 
        wbdrm_loan_performance
    WHERE 
        loan_time BETWEEN '20231101 000000' AND '20231110 000000'
        AND p_day_id = date_sub(getdate(), 1)
    GROUP BY 
        loan_time) e ON a.login_date = e.loan_time;