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MySQL 8.0 Reference Manual
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12.4.4 Assignment Operators

Table 12.6 Assignment Operators

Name Description
:= Assign a value
= Assign a value (as part of aSETstatement, or as part of theSETclause in anUPDATEstatement)

  • :=

    Assignment operator. Causes the user variable on the left hand side of the operator to take on the value to its right. The value on the right hand side may be a literal value, another variable storing a value, or any legal expression that yields a scalar value, including the result of a query (provided that this value is a scalar value). You can perform multiple assignments in the sameSETstatement. You can perform multiple assignments in the same statement.

    Unlike=,:=operator is never interpreted as a comparison operator. This means you can use:=in any valid SQL statement (not just inSETstatements) to assign a value to a variable.

    mysql> SELECT @var1, @var2; -> NULL, NULL mysql> SELECT @var1 := 1, @var2; -> 1, NULL mysql> SELECT @var1, @var2; -> 1, NULL mysql> SELECT @var1, @var2 := @var1; -> 1, 1 mysql> SELECT @var1, @var2; -> 1, 1 mysql> SELECT @var1:=COUNT(*) FROM t1; -> 4 mysql> SELECT @var1; -> 4

    You can make value assignments using:=in other statements besidesSELECT, such asUPDATE, as shown here:

    mysql> SELECT @var1; -> 4 mysql> SELECT * FROM t1; -> 1, 3, 5, 7 mysql> UPDATE t1 SET c1 = 2 WHERE c1 = @var1:= 1; Query OK, 1 row affected (0.00 sec) Rows matched: 1 Changed: 1 Warnings: 0 mysql> SELECT @var1; -> 1 mysql> SELECT * FROM t1; -> 2, 3, 5, 7

    While it is also possible both to set and to read the value of the same variable in a single SQL statement using the:=operator, this is not recommended.Section 9.4, “User-Defined Variables”, explains why you should avoid doing this.

  • =

    This operator is used to perform value assignments in two cases, described in the next two paragraphs.

    Within aSETstatement,=被当作一个n assignment operator that causes the user variable on the left hand side of the operator to take on the value to its right. (In other words, when used in aSETstatement,=is treated identically to:=.) The value on the right hand side may be a literal value, another variable storing a value, or any legal expression that yields a scalar value, including the result of a query (provided that this value is a scalar value). You can perform multiple assignments in the sameSETstatement.

    In theSETclause of anUPDATEstatement,=also acts as an assignment operator; in this case, however, it causes the column named on the left hand side of the operator to assume the value given to the right, provided anyWHEREconditions that are part of theUPDATEare met. You can make multiple assignments in the sameSETclause of anUPDATEstatement.

    In any other context,=被当作一个comparison operator.

    mysql> SELECT @var1, @var2; -> NULL, NULL mysql> SELECT @var1 := 1, @var2; -> 1, NULL mysql> SELECT @var1, @var2; -> 1, NULL mysql> SELECT @var1, @var2 := @var1; -> 1, 1 mysql> SELECT @var1, @var2; -> 1, 1

    For more information, seeSection 13.7.6.1, “SET Syntax for Variable Assignment”,Section 13.2.13, “UPDATE Statement”, andSection 13.2.11, “Subqueries”.