LeftJoinAcross
LeftJoinAcross[{a1,a2,…},{b1,b2,…},keyspec]
gives a list of associations obtained by joining those pairs of associations ai and bj in which the values specified by keyspec match, including all unmatched entries from ai.
LeftJoinAcross[tab1,tab2,keyspec]
joins two tabular objects according to keyspec, including all rows from tab1.
LeftJoinAcross[prefix1obj1,prefix2obj2,keyspec]
prefixes the keys in obji with prefixi using ExtendedKey[prefixi,ckeyij].
LeftJoinAcross[obj,keyspec]
represents an operator form of LeftJoinAcross that can be applied to another object.
Details
- LeftJoinAcross[obj1,obj2,keyspec] is the analog of SQL LEFT JOIN, joining all rows in obj1 to the rows from obj2 in which the values by keyspec match.
- LeftJoinAcross[obj1,obj2,keyspec] is equivalent to JoinAcross[obj1,obj2,keyspec,"Left"]
- In JoinAcross[{a1,a2,…},{b1,b2,…},keyspec], keyspec can be one of the following:
-
key join ai and bj, when the value associated with key is the same {key1,key2,…} all the values associated with each keyi need to be the same in each matched pair keyakeyb the value associated with keya in ai must be the same as the value associated with keyb in bj {keya1keyb1,keya2keyb2,…} match the value associated with keyai in ai with the value associated with keybj in bj func join the pairs ai, bj whenever func[ai,bj] is True - In LeftJoinAcross[{a1,a2,…},{b1,b2,…},keyspec], keyspec can effectively be replaced by the following functions:
-
key #1[key]===#2[key]& {key1,key2,…} #1[key1]===#2[key1]&[key2]===#2[key2]&&…& keyakeyb #1[keya]===#2[keyb]& {keya1keyb1,keya2keyb2,…} #1[keya1]===#2[keyb1]&[keya2]===#2[keyb2]&&…& - Key specifications in LeftJoinAcross must be wrapped in Key unless they are strings.
- LeftJoinAcross effectively generates a filtered version of Flatten[Outer[Join,alist,blist]].
- When a corresponding value is absent, Missing["Unmatched"] is used in its place.
- LeftJoinAcross[{a1,…},{b1,…},{}] and LeftJoinAcross[{a1,…},{b1,…},True&] are effectively the same as Outer[Join,{a1,…},{b1,…}].
- LeftJoinAcross[obj2,keyspec][obj1] and obj1 // LeftJoinAcross[obj2,keyspec] are both equivalent to LeftJoinAcross[obj1,obj2,keyspec].
- The option KeyCollisionFunction can be given to specify what to do with elements of ai and bj that are not specified to be joined but nevertheless have the same key.
- Possible settings for KeyCollisionFunction include:
-
None drop both elements Left include only the left element (default) Right include only the right element func apply func to generate a pair of new keys
Examples
Basic Examples (3)
Join two lists of associations that have the same value for the specified key, keeping all keys from the first:
Join across two Tabular objects, including all rows from the first:
Use the operator form of LeftJoinAcross:
Text
Wolfram Research (2025), LeftJoinAcross, Wolfram Language function, https://reference.wolfram.com/language/ref/LeftJoinAcross.html.
CMS
Wolfram Language. 2025. "LeftJoinAcross." Wolfram Language & System Documentation Center. Wolfram Research. https://reference.wolfram.com/language/ref/LeftJoinAcross.html.
APA
Wolfram Language. (2025). LeftJoinAcross. Wolfram Language & System Documentation Center. Retrieved from https://reference.wolfram.com/language/ref/LeftJoinAcross.html