Mysql：从表中选择 * 其中 col IN (null, "") 可能没有 OR
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Mysql: select * from table where col IN (null, "") possible without OR
提问by pvgoddijn
is its somehow possible to do a select for empty strings and NULL values in Mysql without using or?
是否可以在不使用 or 的情况下对 Mysql 中的空字符串和 NULL 值进行选择？
this:
这个：
select * from table where col IN (null, "");
doesnt work, it ignores the null (or possibly matches it with the string 'null'
不起作用，它忽略空值（或可能将它与字符串“空”匹配
thanks, P.V. Goddijn
谢谢，PV Goddijn
回答by Quassnoi
SELECT *
FROM mytable
WHERE COALESCE(col, '') = ''
Note, however, than ORquery will be much more efficient if the column is indexed:
但是请注意，OR如果对列进行索引，则查询效率会更高：
SELECT *
FROM mytable
WHERE col = '' OR col IS NULL
This will use ref_or_nullaccess path on the index.
这将使用ref_or_null索引上的访问路径。
If you need to select from a list of values along with NULLs, just put all not-null values into the list and add a single OR IS NULLcondition:
如果您需要从值列表中选择NULLs，只需将所有非空值放入列表并添加一个OR IS NULL条件：
SELECT *
FROM mytable
WHERE col IN ('val1', 'val2', 'val3') OR col IS NULL
This will use an index on colas well.
这也将使用索引col。
