编写查询的开发人员没有很好地了解SQL应用程序(包括单个查询和多用户系统)的快慢。这包括对以下方面的无知:

physical I/O minimization strategies, given that most queries' bottleneck is I/O not CPU perf impact of different kinds of physical storage access (e.g. lots of sequential I/O will be faster than lots of small random I/O, although less so if your physical storage is an SSD!) how to hand-tune a query if the DBMS produces a poor query plan how to diagnose poor database performance, how to "debug" a slow query, and how to read a query plan (or EXPLAIN, depending on your DBMS of choice) locking strategies to optimize throughput and avoid deadlocks in multi-user applications importance of batching and other tricks to handle processing of data sets table and index design to best balance space and performance (e.g. covering indexes, keeping indexes small where possible, reducing data types to minimum size needed, etc.)

把东西放在临时表中，特别是那些从SQL Server切换到Oracle的人有过度使用临时表的习惯。只需使用嵌套的选择语句。

我最不喜欢的是

Using spaces when creating tables, sprocs etc. I'm fine with CamelCase or under_scores and singular or plurals and UPPERCASE or lowercase but having to refer to a table or column [with spaces], especially if [ it is oddly spaced] (yes, I've run into this) really irritates me. Denormalized data. A table doesn't have to be perfectly normalized, but when I run into a table of employees that has information about their current evaluation score or their primary anything, it tells me that I will probably need to make a separate table at some point and then try to keep them synced. I will normalize the data first and then if I see a place where denormalization helps, I'll consider it. Overuse of either views or cursors. Views have a purpose, but when each table is wrapped in a view it's too much. I've had to use cursors a few times, but generally you can use other mechanisms for this. Access. Can a program be an anti-pattern? We have SQL Server at my work, but a number of people use access due to it's availabilty, "ease of use" and "friendliness" to non-technical users. There is too much here to go into, but if you've been in a similar environment, you know.

对于存储时间值，应该只使用UTC时区。不应使用当地时间。

使用SQL作为美化的ISAM(索引顺序访问方法)包。特别是嵌套游标，而不是将SQL语句组合成一个更大的语句。这也算“滥用优化器”，因为实际上优化器能做的不多。这可以与非准备语句结合使用，以获得最大的效率:

DECLARE c1 CURSOR FOR SELECT Col1, Col2, Col3 FROM Table1

FOREACH c1 INTO a.col1, a.col2, a.col3
    DECLARE c2 CURSOR FOR
        SELECT Item1, Item2, Item3
            FROM Table2
            WHERE Table2.Item1 = a.col2
    FOREACH c2 INTO b.item1, b.item2, b.item3
        ...process data from records a and b...
    END FOREACH
END FOREACH

正确的解决方案(几乎总是)是将两个SELECT语句合并为一个:

DECLARE c1 CURSOR FOR
    SELECT Col1, Col2, Col3, Item1, Item2, Item3
        FROM Table1, Table2
        WHERE Table2.Item1 = Table1.Col2
        -- ORDER BY Table1.Col1, Table2.Item1

FOREACH c1 INTO a.col1, a.col2, a.col3, b.item1, b.item2, b.item3
    ...process data from records a and b...
END FOREACH

双循环版本的唯一优点是，您可以很容易地发现表1中值之间的中断，因为内部循环结束了。这可能是控制中断报告中的一个因素。

此外，应用程序中的排序通常是不允许的。

回复:使用@@IDENTITY代替SCOPE_IDENTITY()

两者都不能用;使用输出代替

参见https://connect.microsoft.com/SQLServer/feedback/details/328811/scope-identity-sometimes-returns-incorrect-value