問題描述
我有兩張桌子.tbl_names 和 tbl_section 都包含 id 字段.我如何選擇 id 字段，因?yàn)槲铱偸鞘盏酱隋e(cuò)誤:
1052: 字段列表中的列 'id' 不明確這是我的查詢:
SELECT id, name, section從 tbl_names, tbl_section哪里 tbl_names.id = tbl_section.id我可以選擇所有字段并避免錯(cuò)誤.但這會(huì)浪費(fèi)性能.我該怎么辦?
SQL 支持通過在引用前加上全表名來限定列:
SELECT tbl_names.id, tbl_section.id, name, section從 tbl_names在 tbl_section.id = tbl_names.id 上加入 tbl_section...或表別名:
SELECT n.id, s.id, n.name, s.sectionFROM tbl_names n加入 tbl_section s ON s.id = n.id表別名是推薦的方法——為什么要輸入更多的內(nèi)容?
為什么這些查詢看起來不同?
其次，我的回答使用 ANSI-92 JOIN 語法(你的是 ANSI-89).雖然它們執(zhí)行相同的操作，但 ANSI-89 語法不支持外部連接(RIGHT、LEFT、FULL).ANSI-89 語法應(yīng)該被視為已棄用，SO 上有很多人不會(huì)投票支持 ANSI-89 語法來加強(qiáng)這一點(diǎn).如需更多信息，請(qǐng)參閱此問題.>
I have 2 tables. tbl_names and tbl_section which has both the id field in them. How do I go about selecting the id field, because I always get this error:
1052: Column 'id' in field list is ambiguous
Here's my query:
SELECT id, name, section
FROM tbl_names, tbl_section
WHERE tbl_names.id = tbl_section.id
I could just select all the fields and avoid the error. But that would be a waste in performance. What should I do?
SQL supports qualifying a column by prefixing the reference with either the full table name:
SELECT tbl_names.id, tbl_section.id, name, section
FROM tbl_names
JOIN tbl_section ON tbl_section.id = tbl_names.id
...or a table alias:
SELECT n.id, s.id, n.name, s.section
FROM tbl_names n
JOIN tbl_section s ON s.id = n.id
The table alias is the recommended approach -- why type more than you have to?
Why Do These Queries Look Different?
Secondly, my answers use ANSI-92 JOIN syntax (yours is ANSI-89). While they perform the same, ANSI-89 syntax does not support OUTER joins (RIGHT, LEFT, FULL). ANSI-89 syntax should be considered deprecated, there are many on SO who will not vote for ANSI-89 syntax to reinforce that. For more information, see this question.
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