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Comments on How to create a MySQL generated column that uses a join in a concat?

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How to create a MySQL generated column that uses a join in a concat?

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I have a column that is a concatenation of 5 other columns plus a join to a different table.

UPDATE db.a
JOIN db.b ON fk_b = b.pk
SET concat_field = CONCAT(field1,field2,field3,b.field,field4,field5)

What I would like to do is set the concat_field to a generated column,

concat_field VARCHAR(255) GENERATED ALWAYS AS (CONCAT(fields)) stored,

However while it's easy to do with a straight concat the join is giving me trouble.

How would I turn the update statement into a generated field?

mysql generated-columns
posted over 3 years ago
3y ago by Alexei‭
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Charlie Brumbaugh‭
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General comments (3 comments)
General comments
Culyx‭ wrote over 3 years ago
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I'm confused are you trying to store the concatenated value into this table? If so why? If you already have the data available, it sounds like something you would just need to display.

Charlie Brumbaugh‭ wrote over 3 years ago
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@Culyx because joining to a concatenated field of 5 columns is faster than to the 5 columns one at a time.

elgonzo‭ wrote over 3 years ago · edited over 3 years ago
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This approach is not going to work (i am still curious about an alternative solution someone might come up with, though). From the doc about generated cols https://dev.mysql.com/doc/refman/8.0/en/create-table-generated-columns.html, first paragraph: "Values of a generated column are computed from an expression included in the column definition." The column definition is part of the table definition, which means there are no join expressions possible at that point...