Constraints:
default value default
non-null not null
unique unique
primary key primary key
auto_increment foreign key foreign key
default value default
non-null not null
unique unique
primary key primary key
auto_increment foreign key foreign key
master table, slave table
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/*
multi-line comment
result set
merging 1. Requires that in the merged table, the type and number of columns are the same
2. UNION, remove duplicate rows
3. UNION ALL, do not remove duplicate rows