Troubleshooting “Illegal mix of collations” error in mysql

Question

Am getting the below error when trying to do a select through a stored procedure in MySQL.

Illegal mix of collations (latin1_general_cs,IMPLICIT) and

Answer 1

MySQL really dislikes mixing collations unless it can coerce them to the same one (which clearly is not feasible in your case). Can't you just force the same collation to be used via a COLLATE clause? (or the simpler BINARY shortcut if applicable...).

Answer 2

This is generally caused by comparing two strings of incompatible collation or by attempting to select data of different collation into a combined column.

The clause COLLATE allows you to specify the collation used in the query.

For example, the following WHERE clause will always give the error you posted:

WHERE 'A' COLLATE latin1_general_ci = 'A' COLLATE latin1_general_cs

Your solution is to specify a shared collation for the two columns within the query. Here is an example that uses the COLLATE clause:

SELECT * FROM table ORDER BY key COLLATE latin1_general_ci;

Another option is to use the BINARY operator:

BINARY str is the shorthand for CAST(str AS BINARY).

Your solution might look something like this:

SELECT * FROM table WHERE BINARY a = BINARY b;

or,

SELECT * FROM table ORDER BY BINARY a;

Answer 3

Very interesting... Now, be ready. I looked at all of the "add collate" solutions and to me, those are band aid fixes. The reality is the database design was "bad". Yes, standard changes and new things gets added, blah blah, but it does not change the bad database design fact. I refuse to go with the route of adding "collate" all over the SQL statements just to get my query to work. The only solution that works for me and will virtually eliminate the need to tweak my code in the future is to re-design the database/tables to match the character set that I will live with and embrace for the long term future. In this case, I choose to go with the character set "utf8mb4".

So the solution here when you encounter that "illegal" error message is to re-design your database and tables. It is much easier and quicker then it sounds. Exporting your data and re-importing it from a CSV may not even be required. Change the character set of the database and make sure all the character set of your tables matches.

Use these commands to guide you:

SHOW VARIABLES LIKE "collation_database";
SHOW TABLE STATUS;

Now, if you enjoy adding "collate" here and there and beef up your code with forces fulls "overrides", be my guess.

Answer 4

If you have phpMyAdmin installed, you can follow the instructions given in the following link: https://mediatemple.net/community/products/dv/204403914/default-mysql-character-set-and-collation You have to match the collate of the database with that of all the tables, as well as the fields of the tables and then recompile all the stored procedures and functions. With that everything should work again.

Answer 5

TL;DR

Either change the collation of one (or both) of the strings so that they match, or else add a COLLATE clause to your expression.

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1. What is this "collation" stuff anyway?

   As documented under Character Sets and Collations in General:

   A character set is a set of symbols and encodings. A collation is a set of rules for comparing characters in a character set. Let's make the distinction clear with an example of an imaginary character set.

   Suppose that we have an alphabet with four letters: “A”, “B”, “a”, “b”. We give each letter a number: “A” = 0, “B” = 1, “a” = 2, “b” = 3. The letter “A” is a symbol, the number 0 is the encoding for “A”, and the combination of all four letters and their encodings is a character set.

   Suppose that we want to compare two string values, “A” and “B”. The simplest way to do this is to look at the encodings: 0 for “A” and 1 for “B”. Because 0 is less than 1, we say “A” is less than “B”. What we've just done is apply a collation to our character set. The collation is a set of rules (only one rule in this case): “compare the encodings.” We call this simplest of all possible collations a binary collation.

   But what if we want to say that the lowercase and uppercase letters are equivalent? Then we would have at least two rules: (1) treat the lowercase letters “a” and “b” as equivalent to “A” and “B”; (2) then compare the encodings. We call this a case-insensitive collation. It is a little more complex than a binary collation.

   In real life, most character sets have many characters: not just “A” and “B” but whole alphabets, sometimes multiple alphabets or eastern writing systems with thousands of characters, along with many special symbols and punctuation marks. Also in real life, most collations have many rules, not just for whether to distinguish lettercase, but also for whether to distinguish accents (an “accent” is a mark attached to a character as in German “Ö”), and for multiple-character mappings (such as the rule that “Ö” = “OE” in one of the two German collations).

   Further examples are given under Examples of the Effect of Collation.

2. Okay, but how does MySQL decide which collation to use for a given expression?

   As documented under Collation of Expressions:

   In the great majority of statements, it is obvious what collation MySQL uses to resolve a comparison operation. For example, in the following cases, it should be clear that the collation is the collation of column charset_name:

   SELECT x FROM T ORDER BY x;
   SELECT x FROM T WHERE x = x;
   SELECT DISTINCT x FROM T;
   

   However, with multiple operands, there can be ambiguity. For example:

   SELECT x FROM T WHERE x = 'Y';
   

   Should the comparison use the collation of the column x, or of the string literal 'Y'? Both x and 'Y' have collations, so which collation takes precedence?

   Standard SQL resolves such questions using what used to be called “coercibility” rules.

   [ deletia ]

   MySQL uses coercibility values with the following rules to resolve ambiguities:

   • Use the collation with the lowest coercibility value.

   • If both sides have the same coercibility, then:

     • If both sides are Unicode, or both sides are not Unicode, it is an error.

     • If one of the sides has a Unicode character set, and another side has a non-Unicode character set, the side with Unicode character set wins, and automatic character set conversion is applied to the non-Unicode side. For example, the following statement does not return an error:

       SELECT CONCAT(utf8_column, latin1_column) FROM t1;
       

       It returns a result that has a character set of utf8 and the same collation as utf8_column. Values of latin1_column are automatically converted to utf8 before concatenating.

     • For an operation with operands from the same character set but that mix a _bin collation and a _ci or _cs collation, the _bin collation is used. This is similar to how operations that mix nonbinary and binary strings evaluate the operands as binary strings, except that it is for collations rather than data types.

3. So what is an "illegal mix of collations"?

   An "illegal mix of collations" occurs when an expression compares two strings of different collations but of equal coercibility and the coercibility rules cannot help to resolve the conflict. It is the situation described under the third bullet-point in the above quotation.

   The particular error given in the question, Illegal mix of collations (latin1_general_cs,IMPLICIT) and (latin1_general_ci,IMPLICIT) for operation '=', tells us that there was an equality comparison between two non-Unicode strings of equal coercibility. It furthermore tells us that the collations were not given explicitly in the statement but rather were implied from the strings' sources (such as column metadata).

4. That's all very well, but how does one resolve such errors?

   As the manual extracts quoted above suggest, this problem can be resolved in a number of ways, of which two are sensible and to be recommended:

   • Change the collation of one (or both) of the strings so that they match and there is no longer any ambiguity.

     How this can be done depends upon from where the string has come: Literal expressions take the collation specified in the collation_connection system variable; values from tables take the collation specified in their column metadata.

   • Force one string to not be coercible.

     I omitted the following quote from the above:

     MySQL assigns coercibility values as follows:

     • An explicit COLLATE clause has a coercibility of 0. (Not coercible at all.)

     • The concatenation of two strings with different collations has a coercibility of 1.

     • The collation of a column or a stored routine parameter or local variable has a coercibility of 2.

     • A “system constant” (the string returned by functions such as USER() or VERSION()) has a coercibility of 3.

     • The collation of a literal has a coercibility of 4.

     • NULL or an expression that is derived from NULL has a coercibility of 5.

     Thus simply adding a COLLATE clause to one of the strings used in the comparison will force use of that collation.

   Whilst the others would be terribly bad practice if they were deployed merely to resolve this error:

   • Force one (or both) of the strings to have some other coercibility value so that one takes precedence.

     Use of CONCAT() or CONCAT_WS() would result in a string with a coercibility of 1; and (if in a stored routine) use of parameters/local variables would result in strings with a coercibility of 2.

   • Change the encodings of one (or both) of the strings so that one is Unicode and the other is not.

     This could be done via transcoding with CONVERT(expr USING transcoding_name); or via changing the underlying character set of the data (e.g. modifying the column, changing character_set_connection for literal values, or sending them from the client in a different encoding and changing character_set_client / adding a character set introducer). Note that changing encoding will lead to other problems if some desired characters cannot be encoded in the new character set.

   • Change the encodings of one (or both) of the strings so that they are both the same and change one string to use the relevant _bin collation.

     Methods for changing encodings and collations have been detailed above. This approach would be of little use if one actually needs to apply more advanced collation rules than are offered by the _bin collation.